While I can understand not wanting a diversity of clients for the support headache that could rapidly become, I don't fully agree that a diversity of clients forces them to be stuck at a lowest common denominator. What prevents them from making the breaking changes necessary for security and features, and letting the devs of 3rd party clients successfully conform or not?
I see it as something like mandating minimum version of TLS or cipher suite. The security flaws of TLS version <1.2 have been documented for some time now. I've had to tell customers more than once we are disallowing use of older insecure protocols to access my employer's services.
Or am I misunderstanding you and actually happen to be agreeing?