- constantly changing things around. UI gets less intuitive for every release.
- slower for every release
- violates IMAP standard by re-using IDs across tags/IMAP folders, risking actual data loss. Example: if you in a real email-client try to delete a email from a single folder, you will also delete this email from all other folders it has been "tagged" in.
- similar IMAP issues with sent emails. Hard to track sent emails from email-client, unless you allow the email-client to explicitly save a copy to sent-emails folder. But then you suddenly have duplicate emails in the web-UI.
- Makes an open standard (internet email) proprietary.
- Hell to integrate with: See above.
I'm sure I could go on, but really. If you still consider GMail best in class, the only possible explanation is that you haven't seen anything else.
They just assume everyone uses the web interface.
More likely, they want you using their app and their web interface, and merely tolerate third party apps. For now.
Google wants you in the Googleverse forever.
Gmail/Gsuite seem to be good email services from my perspective as lay user and occasional admin. Can you expand on why you think they are not good email services?