I think the centralization on mastodon.social is a bit over estimated. According to fedidb.com there are over 1.1M active users in the fediverse. Of these mastodon.social has about 273k. That means mastodon.social has about 1/4 of the active users. The rest are scattered over nearly 43k servers. So I'd say the fediverse is pretty decentralized. That would make it pretty resilient.
This is the real strength of ActivityPub compared to ATProto. I follow a lot of accounts on mastodon, and only a handful of them are on mastodon.social. Globally though looks like 1/4 of the accounts are there.
If the instance goes down, the network stays up and continues to work, minus only the accounts on mastodon.social. This is not the case on bluesky afaik. They got DDosed a few months ago and the whole network was down because of it. https://news.ycombinator.com/item?id=47802330
In ATProto there aren't 'instances', and its technically 'decentralized' but theres really only bluesky.
