We agree here, but this will not necessarily stop the rot. Companies will just approach these communities and offer the leaders massive buyouts, and then flood it with ads (see Twitch).
Communities themselves need mechanisms to punish or vandalize advertisements.
People moved from Facebook to Reddit in the past because it was seen as the more community-centric platform.
This has taken a wild shift over the last 5 years; no one who moved over was hoping for Reddit becoming an ad-centric platform.
Decentralization is not a silver bullet. If lemmy.world hits 1 million users, and then a large corporation buys it, lemmy will be set back 10 years. This is an incentive problem, and no amount of workarounds is going to fix it.