- Even if it were true that "everyone uses Google/Microsoft/Apple/etc", the ones that don't can still send/receive emails from everyone else. - If one of the of the big providers disappear, the system as a whole will continue to function.
As for the few that do have the capacity to setup a mail server, they would only be able to message others that also have the capacity to setup a mail server (again we're assuming no centralized providers and thus no delegation of this task to a trusted party). So maybe the system would still work for this niche group... but then we get into the bigger question: are there entities today that control the Internet and do they have the capacity to shut down systems like email? Tor?
Call him up on the phone or message him on whatsapp or... whatever...
email IS decentralised. Even if we had ONLY gmail, a startup could implement a better email frontend and be successful.
The fact that you don't get to talk with other people only because an big email provider would disappear does not change the fact that email messaging systems are designed to be inter-operable. That is the key point. To try to argue otherwise is insane.