This schema relies on "third-party aggregators / crawlers / search services" for critical parts like comments and content discovery. They are going to be responsible for things like spam filtering, misinformation removal, and blocking accounts which violate TOS.

And those crawlers _have_ to be expensive - after all, you need to check every participating server to see comments. So instead of everyone running mastodon instance on $10/month machine, it's only going to be a few centralized services doing all the work.