I can imagine some sort of car-wash-like partial mouth insertion interface (think "smart cleaner/retainer"), but it would be cost-prohibitive and, likely, not offer any appreciable cleaning benefits.

There are electric-, ultrasonic-, mouthpiece-, and irrigating toothbrushes...

Maybe the experience has not changed for the average person, but alternatives are out there.

I feel like toothbrush UX has improved quite a bit.

I was going to say "are we stuck with the same bicycle UX forever".

Because we've been stuck with the same bicycle UX for like 150 years now.

Sometimes shit just works right, just about straight out of the gate.