Poor people (who still can afford the end devices and an Internet plan) can of course share the costs in a community, or use one of the many free servers, as long as they are aware of the tradeoffs. Beigers not being choosers, and all that.
Federated protocols are not centralized in principle. It might not scale to one user-one server (which probably even Lemmy can’t handle) but if you’re signing up for a central server, you’re doing it wrong™. Don’t do that. The nice thing about Matrix client is that it allows end to end encryption, including groups. So that greatly limits what Mallory can do in principle. As to servers being costly to run, given what documented Synapse requirements are, you’re looking at less than 5 EUR/month for a single server. Which can be shared among several users, obviously. This is in the same range as costs for a monthly VPN.
Self-hosted Matrix is obviously unaffected.
It’s not going to, though. As soon as the tariffs disapper they’ll be impersonating Dory, again.