It was meant to replace email, an open standard sporting many interoperable servers and clients, with something that Google controlled, even though it was theoretically federated.
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.
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.
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.