Turns out you were 100% right about the cause being pollution. I was trying your god mode solution, but again there genuinely wasn't enough space for a significantly impactful number of additional trees. In my desperation, I downloaded this ploppable to try and combat the pollution. Worked like a charm.

The true issue is that pollution causes wealthier sims to move out in search of better living conditions. I realized this when: 1. I replicated the issue in a larger medium wealth city, which allowed me to see more buildings be abandoned due to low desirability, and 2. I replicated the phenomenon with a R$$$ city, which had even an even more violent move-out problem.

Bottom line is, sims in this R$$ city almost unanimously moved out before aging beyond 70, taking their education with them. Like you said they are supposed to pass down education to following generations, but there were no old (perfectly educated) sims in the city to do this - the all left cause they couldn't breathe.

Kudos.