But that only started when gmail launched with a deal that was too good to refuse (a mailbox that was huge for the time, and grew bigger over time). Before that most people just used the free email account they got from their ISP. That also comes without maintenance headache for the user and is at least somewhat less centralized

Yeah but you change your ISP more often (pricing reasons, moving to a different state or country) than your email provider.

Damn, in the last twenty years I've never lived at a place with more than one option for ISP (excluding satellite options).