Everybody I know who still runs their email server (you can get Europeans out of Europe, but you can't get Europe out of the Europeans) complains about Microsoft.

I bet Microsoft is single-handedly responsible for 80+% of complaints about email being impossible to host.

In a federated / pseudo-decentralized network, any provider with 10+% of captive users has an outsized leverage on the entire ecosystem. You don't need a monopoly; as long as you have a monopoly on just a few users that others absolutely need to be able to interact with, you can dictate terms to everybody.