That's not how E2E encryption works. It's not just e-mail administrators updating their systems. Every client would need to support whatever E2E encryption scheme chosen. That's hundreds of individual client applications (potentially thousands counting various versions) that would need to be updated.
Contrast with WhatsApp where they control the entire stack from client to network to server. They can push a single set of updates and everyone has the new hotness.
Stage one is that the old email servers become an end for all of their users. From there services can start shifting to having clients where users control the keys.
>Every client would need to support whatever E2E encryption scheme chosen.
If someone couldn't use gmail with an email client the author of that client would have a big incentive to add support for it else risk their users switching to a client that does.
Contrast with WhatsApp where they control the entire stack from client to network to server. They can push a single set of updates and everyone has the new hotness.