No, I actually don't think so. Both parties in the US shift so there is an equilibrium between them and they both always represent ~50% of the electorate. It's obvious that Trumpism even in 2016 is already incapable of winning an election without both the Electoral College and a strong leftist third party to bolster them, and that its reliance on older voters (both in the general election and in the Republican primary, incidentally) means that it can't hope to survive for long. When it is decisively defeated (and this is a matter of when, not if, unless the people turning 18 now become very staunch advocates, which seems unlikely demographically), the Republicans will go through a period of figuring things out, but they'll be back.
It's a little late on this front, don't you think? Decades late, in fact.
What is your point?
You keep talking about 'young republicans', but the fact is that very few young republicans exist. Most youths are bernie-supporting democrats, now when those youths grow up are they more likely to become doctrinaire conservatives or populists?
That same argument literally goes for Bush in 2000 with Nader.
The same argument doesn't quit go for Bush 2000 with Nader. Bush wasn't as detested among those who didn't vote for him as Trump is now. Bush started at 65/20 in approval; Trump started at 45/45. Making gains among those who disliked him was an option for Bush. It seems unlikely to be one for Trump.
As for young Republicans, I'm talking about voters under 45 who voted in the Republican primaries in 2016, and to a lesser extent also 2012. These people exist and their preferences in both years veered wildly away from the rest of the party, which outvoted them. Trump tended to be very, very weak among them, and they generally shifted between whoever was the strongest anti-Trump candidate without regard to ideology, voting for Cruz/Rubio/Kasich in different states.
"Doctrinaire conservatives" is not a good summary either -- these people also disliked Romney in 2012, giving first place to Paul and second to whoever the anti-establishment flavor of the day was (either Santorum or Gingrich), with Romney in third in many places he won. My own assessment is that they tend to be isolationist, reflexively anti-establishmentarian, and with a strong fiscally conservative streak, and divided on social issues, with there being a faction that doesn't care and another that remains very, very right-wing.
Trump failed to appeal to these people in the primary
at all. To the point that they demonstrated that they liked every other wing in the party better. And it's not like there's a demographic amongst young Democrats who'd find Trumpism appealing. So, yeah, I don't think Trumpism is particularly long for the world.
