Why do you think Google Wave was user-hostile?
After Wave failed, they doubled down on making Gmail into more of a nonstandard product with reduced interoperability (now requiring Gmail API instead of standard IMAP) and increasingly, embrace & extend functionality such as email expiration dates.
Is something not working with gmail's imap support?
https://news.ycombinator.com/item?id=16951363
https://www.dvratil.cz/2014/06/improved-gmail-integration-in...
https://productforums.google.com/d/msg/gmail/9A5oYELFbu8/7iq...
https://github.com/mscdex/node-imap/issues/71
https://freron.lighthouseapp.com/projects/58672/tickets/1247...
Sure you can still access Gmail through IMAP, but if it works differently enough that using a standard IMAP client feels cumbersome and unfamiliar, is it really anything else than a vehicle to tell people that they should really just use the "better" Google product directly?
That said, my original wording of "requiring" the Gmail API was poor and I should have phrased it more accurately.