Decentralized identities are pretty important. It solves the following problem,

"My followers can't read my content because their admin says so".

Obviously hosting your own instance as you alluded to doesn't solve that problem because the admin of your followers can ban your instance. It's better for users to push and pull from multiple instances than to have a home instance like the fediverse. This is kinda complicated to wrap your head around if you are new to nostr and the fediverse. But in nostr you can switch instances without changing identities. This makes it easy to pack up and go if your admin is shit. Without losing followers and stuff.

"Conceivably I can spend a lot of effort posting to your community here, only for you to get fed up with me and yeet me off it. Isn't that still a petty dictator?"

Yes I can make it so I don't approve any of your post in communities I make. And if another moderator doesn't agree I can remove them because I am the community creator. But.... You can just open your own community and if your content is good people will subscribe to it. I can't tell other people not to follow your public key. There is no concept of defederation. Nostr's chat functionality works for communities with no moderation. The moderated community feeds are for currated content. The job of a moderator should be to keep things on topic and interesting enough to keep people there and if they want to leave they should be able to leave without losing followers. Nostr allows for this, the fediverse and centralized social media doesn't.

We will get clients that display multiple communities in a single timeline. We will get better spam control. Just give it a year or two. We can make quick patches to deal with sudden spam inflows.