A lot of their kids (my peers) ended up unmarried and childless as per this article. So in a way those parents got punished by evolutionary forces for not being careful enough about their kids. I can guarantee you that those of my generation who "made it" through that filter are vigilant to ensure it doesn't happen to our kids.
But college is not the be-all and end-all of someone's political evolution. Buying a home and having a family also influences someone's political opinions, broadly in the more conservative direction.
What I think sounds plausible is that the rise of women pursuing post-graduate degrees brings down birth rates. If a woman isn't done with her academic pursuits until she's 30, she'll unquestionably be less likely to have kids as a matter of biology.
Moreover, if it's extremely difficult to buy a house and start a family until you're in your mid-30s, that's going to keep people more liberal for longer.
So I don't think the basic dynamics have changed, but the timelines have. When people have a house and family, they have things to lose and act more self-interested. When they don't, they don't, they're much happier to entertain proposals for vast societal change.
I think the right has been evolving higher memetic immunity, which is causing this strategy to become less effective over time.
Increases in homeschooling and private religious schools – school vouchers in the US really help with that. Reduced rates of cross-political friendship, dating and marriage. Increased geographic sorting based on ideology. Social media echo chambers. The "right-wing media ecosystem" (see e.g. Libs of Tiktok) is a lot more engaging than it was 40 years ago. Internet filtering (some religious groups pressure even adults to install it.) Increasing political pressure on universities to moderate their politics reduces their effectiveness at transmitting left-wing politics to students, meanwhile right-leaning alternative tertiary institutions are growing.
Also, odds of political defection is partially determined by personality traits, which are partially genetic. This creates selective pressure to reduce the frequency of defection-promoting alleles in right-leaning populations across generations, which is a genetic rather than memetic factor predicting that conservative retention rates will rise over decades to come. See https://pmc.ncbi.nlm.nih.gov/articles/PMC3125629/ which discusses this with respect to genes for religiosity, which is heavily overlapping with (albeit not quite the same as) political conservatism.
Ahh. Brood parasitism. Very tricky.
That does not mean that it will work against all of them. Some high-fertility groups of today don't seem to be particularly prone to losing their members to left-wing or even just generic secular persuasion: there are very few ex-Amish or ex-Haredi leftists, and some, but not very many, ex-Muslim and ex-Mormon leftists.
Your counterexamples are indeed the succesful defenders, the ones the right could learn from. The Amish (the only successful resisters of brood parasitism I'm directly familiar with), don't have to worry about capital mapping their offspring's desire because they have created an effective cultural barrier from it. No doubt many young Amish would find superhero media alluring, but movies, TV, and phones need electricity, which they have forbidden from their personal lives. More generally, the hierarchy of God, family, work, then finally self is fundamental and encoded into the child's being. To electrify your bedroom is to no longer be Amish, which has a lot more friction than drifting from your parents' mainstream conservatism.
I don’t think this means what you think it means
Your notion of “the left” as some sort of… swirl of evil linguistic specters that prey on the good and civilized flesh-and-bone right is better suited to post-OT III Scientology than Lacan. If you want a philosophy that your ideological detractors are literal evil ghosts they have recruiting offices in most major cities.
In fact, that's a very real threat if you're the left, since if this mechanism reverses, now you've got a real reproduction problem.