If you don't host it, how can you be sure you own it?
"We get all of this great stuff, and for free! I mean, how do they manage it?"
ZDNet: Yandex said it caught an employee selling access to users’ inboxes
Another reason why ownership of your own data, and its protection through the means of encryption, is paramount in importance. Technology may mask this point, and abstract away who else has access to the stuff you upload, but the importance has been and always will be there.
And by the way, this is not the kind of stuff that can get guaranteed protection with the "Protonmail approach" of "supposedly end-to-end encryption" either: even the German "champion" provider Tutanota was forced to backdoor one of its users too.
Part of the reason why this problem is allowed to exist is how email itself is structured: thanks to spam, it's practically impossible to self-host, so you have to defer it to the "Big Three." I don't know the solution to this yet, but I presume it has to do with going with a peer-to-peer model, where you own both the data and the infrastructure required to use it.