1) Walker and Perry bombing out wasn't a very big surprise. Perry probably wasn't to almost any observer. For some reason Walker was. I was surprised at how fast he flamed out (he was leading polls all the way back in July for some reason), but that he did so at all was not that disturbing or unpredictable and I was suggesting as much months ago. He was not a very skilled politician and very clearly did not have good instincts for national politics or foreign policy questions of the sort that do not trouble a Governor's campaign but will bedevil a Presidential campaign.
What he always reminded me of was Tim Pawlenty's failed run in 2012: he was a conservative governor from a state neighbouring Iowa. That's it. That was his selling point. That did not work out so well for Pawlenty. There was never much reason to think it would with Walker. Union-busting is a state issue. It doesn't have much relevance or resonance at the national level, even among conservative politicians and voters. Since that seems to be the only significant campaign story Walker felt he could run on, that isn't going to get him very far. Not surprisingly it flamed out and did not work.
2) Rubio still looks to me like the most likely GOP nominee. I would feel pretty confident betting on it at least. There is still a significant "anybody but a Bush" attitude that pervades the GOP base, in a strong way that was not the case with Romney 4 years ago. There is still ambiguity about the direction of the party and its ideological positioning on issues like immigration that I think there are risks. But those risks won't end up with Trump or Cruz or some such at the head of the class instead either. Unlike those two, Rubio actually has establishment support and will probably rally more of it with Walker out of the way. He also hasn't shriveled up, been incapable of answering questions, or seemed like a blithering idiot like most of his rivals (in particular Bush and Walker). I don't agree with very much of any of his policy agenda. But if I had to guess, that's still the guy I would bet would be the nominee next year.
3) Sanders percolates up now and again in my feed of social media still, in part because I know a lot of liberals it seems. I think I have identified why I just don't get him as being all that interesting.
a) a Presidential campaign that all but ignores foreign policy, to the point of being pretty actively dismissive of it as a topic in interviews, to me is really not a serious campaign. This is on top of Sanders' positions on trade being very retrograde (like dismissed 250 years ago, even though they keep popping up in far left political talks). I'd like to think Sanders has somewhat agreeable views on IR, but given how dismissive his campaign tends to be of the topic, I'm ambivalent that this is actually the case enough. A domestic centered agenda, particularly an economic one, just doesn't interest me as a voter because most of that will require action of Congress or the Supreme Court. And it seems extremely unlikely that Congress or the Supreme Court will be radically altered sufficiently to enact most or even any significant portion of a Sanders agenda. By contrast, US Presidents can do a lot on the foreign policy scene. I wish more Senators or voters took this as something worth considering, but if Sanders wants to run on a domestic agenda, he should really stay in the Senate. That's where he can do the most damage.
b) The biggest primary election problem Sanders has had is that non-white voters, which account for about half of the Democratic electorate nationally, aren't all that interested in him. It has always been possible he could fix this, but I do not think he's capable of it based on his primary election positions and campaign fervor. The main problem, which I tend to agree with, is that Sanders' explicitly addresses questions of inequality as in and of itself a problem which results in racism. Rather than racism being in and of itself a cause of inequality. Police officers are not beating up or shooting unarmed black kids because of inequality. There's a case that kids from poor neighborhoods cannot get through college because of inequality, but that case isn't resolved by making college free, which for whatever reason remains a popular campaign talking point for Sanders. That case is resolved by improving the K-12 environments in which many minorities are living, and also by resolving or addressing strongly the issues of criminal justice reform more broadly on which police brutality is merely the tip of several icebergs of our country's problems on these questions. Sanders, and importantly, many Sanders supporters, seem to want to focus on questions of economic inequality first as they impact the white middle class rather than deal with the lower end of the pie and why those slices are distributing oddly. This sort of focus can carry states like Vermont, New Hampshire, or Iowa. States where Democratic politics and the state as a whole are dominated by white middle class voters. It will do him no credit in Nevada or Colorado or Florida or South Carolina. These are separable problems, and they are problems which demand distinct attention if someone is going to address them. Feeling like the problems of non-white voters are second class status is not a way to interest those voters.
More to the point, most of the agenda of a movement like BLM can get reasonably far with the support of only a President, rather than a Congressional action being required, as they pertain to DoJ authority and various executive branch programs and agencies in some way. A President cannot and will not solve racism (or for that matter, economic inequality), but can do some things to hammer state and local police forces to behave by siccing lawyers and federal authorities and judges on them, conducting investigations, requiring reforms, and tying those reforms to federal dollars and funding that some local forces may rely upon. All of which are things liable to help the lives of people much, much sooner than long-term economic reforms with the problems many minorities experience when dealing with police and law enforcement in general. The lack of focus on this as an immediate priority to eliminate easy sources of human suffering and brutality and misconduct is not lost on those who are suffering. There are sound political reasons not to focus on these issues (eg, white middle class voters tend not to care about them), but that only makes the division worse and more evident.
c) Inequality in general is the main theme on which Sanders seems to dominate his focus. I don't share this focus, but if I did, I'm not sure I would diagnose what he has as methods to resolve it. Min wage law doesn't seem to have a significant impact in the empirical literature and if anything might actually be harmful. It could be helpful too, but in general probably won't do very much about the problem of "fat cats on Wall Street" or "greedy CEOs", which is where almost all of the inequality disparity in the US stems from, and won't do very much about the plight of the poor, or the income traps of the structure of our existing welfare and social safety nets (where marginal tax rates can exceed 100% and people on net lose money by making more). It is not a very exciting policy change to demand as a result. College education is largely still reasonably affordable, even as the cost has grown substantially (in part because of federal subsidies). Reforms to make it free to the public are largely a give away to the fairly well off people who privately gain from attaining a college education already and do little about the basis on which a college education has grown in importance over a mere high school diploma in the first place, namely that too many jobs have become credentialized and licensed to require one for no apparent reason, and that a K-12 education isn't seen as satisfactory for many jobs. One reason it is not is that it is sometimes not seen as satisfactory for many colleges either. And then the another focal point seems to have been campaign finance laws, with the suggestion being that rich people are buying elections. This may be true that rich people can spend somewhat more on elections, but one reason rich people are "buying" elections is that richer people will vote. A lot more often than poorer people. There are a lot of reasons for that. Most of them do not have to do with "rich people can buy elections" in general. That is not going to fix American democracy or voter turnout if those are perceived to be imperiled by the amount of money now being spent on national politics.
A generally middle class populist campaign that focuses on these, as well as some of the protectionist themes, marginally anti-rich/anti-capitalist rhetoric, etc is not likely to attract much interest from me in the first place. But if it comes away from a putative problem like inequality with bad solutions, or the wrong solutions, or solutions to problems we don't have, and so on, it is really not going to get my attention in a good way.