...and then there's the Zoomers! Seriously is anyone under 40 a Republican?
There are under-40 Republicans, but you won't find any Republicans under about 28 or so. I know exactly one in real life, and I've literally never lived anywhere other than very white middle-class suburbs in solidly Republican states.
Also, I think the high Democratic turnout of the last few years won't go away when Trump is gone. For a long time we will be very determined to not let another Trump happen.
This is called selection bias: my circle of friends in our 20s is largely right-wing. I don't doubt that the generation as a whole is quite left-wing, though. (Though 2016 exit polling showed reverse age gaps in rural white areas; Democrats are being held up by margins with old folks in states like Iowa and Maine. Nationally the GOP as currently constituted is doomed across most of the country, though.) What will probably happen here is that the issues motivating the parties will shift over time; since their current positioning is just not an electoral winner Republicans are unlikely to maintain it.
Anyway, turnout in the United States virtually always falls under a Democratic presidency and rises during a Republican one, and structural factors are largely pushing it downward and not upward (like the collapse of local news and organized religion, which drove turnout in the past). The Trump bubble in mass political interest is probably a bubble, even if it pops slowly rather than quickly. And I'm inclined to think it'll pop quickly -- the 2008 enthusiasm popped in
months rather than years, and congressional numbers mean it'll be harder for Biden to mount an aggressive agenda than it was for Obama even in the most optimistic landslide.
I doubt GW would have won the popular vote in 2004 if it weren't for 9/11.
Under the 2016 alignment, assuming high polarization, the GOP doesn't need a national popular vote victory for a
filibuster-proof Senate majority, and can hold a bare majority way out to huge Democratic victories (note that Biden is winning by 10 points right now per 538, but the median Senate result is just 51D-49R; Republicans holding the Senate remains quite possible). This is a bad metric under our current constitutional system.
