The bristles wear out. Their heads wear out but they also become soft from all the vibration so they dont't push as hard on the teeth. Also gunk might accumulate depending on how careful you rinse it.

Still better than a manual brush even in that state.

No, it doesn't appear so. It seems the bristles do wear out somehow, to me it appears that they simply become less stiff over time. The change to a new brush head is noticeable.

This should be relatively easy to verify. One could take a new brush head and forward its counter to the limit, directly comparing it to a new unmodified brush.

This is a good question, but people are pretty sensitive to pitch changes, I think we would detect the motor slowing down.

It would be super easy to reveal as well. A family member with the same toothbrush, your head finishes first. Motor slows down, pitch goes down. Compare the two. Replace the old head, now they're the same. Scummy practice revealed. Scandal.

That said. I'm not totally sure on the mechanism that all electric toothbrushes use.

It's much harder to detect subtle amplitude changes.