Also, a percentage of those millennials who age up will trend to the right as they come in to more financial stability and continue to be old on the idea that if you're stable enough sometime you might get super stable and hit it real big, at which point Republican tax laws will benefit them. That idea is so baked in to the myth of the American Dream that, as passionate as you may see younger generations now (of which I still technically qualify, I guess), it's likely to fade as they shift demographics. I figure there's a trend of people getting more conservative leaning as they age, but that's just speculation on my end.
In regard to not seeing a Democratic candidate to beat Trump, I think the primary is a byproduct of that, but not the reason I see things that way. Maybe if they can mobilize voters, perhaps, but at that point you're looking at Biden (presumably, he seems to think that he's the Great White Hope or something and that all non-white voting blocs are ready to come out in droves to support him), Sanders, and Warren, both of which do seem to have pretty dialed in bases. Not sure they outweigh Trump's base though, which will bring it to swing voters/ones who are also not apathetic. Someone mentioned after the last debate, talking either to or about Pete, I believe, that it's hard to be sure he'd be ready to stand head to head on a debate stage with Trump because it won't be like anything he's seen at the primary, and I get that sense as well, with him especially. Biden is so dialed in on rhetoric that I also feel like Trump's theatricality has a pretty decisive advantage on him there, and he's basically been prepping attacks on Warren since before she even announced candidacy, so, mixed in with how he feeds on hate, racism, and misogyny, that I also find it difficult to see him not coming out as a decisive 'winner' in the perception after their debates. Maybe Sanders can match him, since he also dabbles in the theatricality, but even that I'm not sure about, though of the people left he's the one I feel who has the biggest advantage in that regard.
Of course the real solution is shutting everything down on election day and making the process so simple/uniform that a need to mandate voting isn't even a necessity, but, obvi, neither of those will happen. I've voted in every election/primary/whatever at each level since I turned 18, even though I can say that my vote has felt important only once or twice, but I imagine that's a hard sell for most people and the fight against apathy is the real one that needs to be won.