I agree that subcommunities are a useful feature. That only proves my point, though. The positive example you give, Discord servers, are a lot more like subreddits than they are like Mastodon servers. You can join multiple of them, the power of the moderator is local and doesn't affect your account as a whole. Discord and Reddit are of course centralized services, but in theory there's nothing preventing a decentralized system from having a similar feature.