The end-to-end encryption is only between protonmail addresses, in practice when you email people with gmail/hotmail/yahoo etc. it doesn't matter if protonmail can't read the e-mail, the other party can. (Their solution for that is to send an e-mail that contains a password-protected link with the actual message [0], I find this procedure inconvenient.)
Gmail could be as secure as Protonmail by using PGP yourself [1]. And then you can keep your desktop mail client.
Also Google won't need to pay criminals if they're DDOS attacked like Protonmail in the past: https://www.forbes.com/sites/thomasbrewster/2015/11/05/proto... (this is when I gave up on Protonmail)
[0]: https://protonmail.com/support/knowledge-base/encrypt-for-ou...