

They could expand the Xbox controller API feature set tomorrow
No, they couldn’t. There’s over 20 years of legacy hardware and software that expects Xinput data to be returned exactly in this format:
typedef struct _XINPUT_GAMEPAD {
WORD wButtons;
BYTE bLeftTrigger;
BYTE bRightTrigger;
SHORT sThumbLX;
SHORT sThumbLY;
SHORT sThumbRX;
SHORT sThumbRY;
} XINPUT_GAMEPAD, *PXINPUT_GAMEPAD;
Changing any of that would break every single xinput controller and game made in the last 20 years. Modifications require a new API, which is exactly what GameInput is.

Agreed :)
And no matter how they do it, it’s clear they need to make changes; “everything is an Xbox controller and now just works” actually was pretty nice solution 20 years ago to fix the complicated mess that was PC gamepads with DirectInput, but it is very much outdated idea today.
Thankfully it appears changes are happening that might solve the issue in the near future, and all we can really do is wait and see.