I am pretty sure if Google disappeared we were going to see a huge drop in the email traffic. How would I get to know the new email address of my friend who we only talked using Gmail for example?
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.