I have not observed that people on lemmy are overall more intelligent than people on reddit.

Because it's literally the same userbase, 99% of people here came from Reddit for one reason or another. It's a fact that any community that grows too mainstream starts to decline in quality, so if you think Lemmy already has too much garbage you really don't want it to become mainstream. Proportionally we have way less spam, astroturfing and just idiotic bullshit being posted outside of meme subs here, attracting more users would make all of these things more prevalent and no amount of decentralization would solve that unless people started to mass defederate, but at that point why even bother with activitypub in the first place. It's kind of like the people in Beehaw that not only chose this platform but also want to federate with other instances but they're constantly crying and complaining about other instances, threatening to leave the platform... Like, why even bother with activitypub?