They could be at every priority seat.
Stop digging!
@TUC has already explained so I won't repeat the same point, but your idea of having buttons to trigger announcements is unworkable, not to mention that it will add additional costs to the system.
It would likely be counterproductive anyway; what are the chances of someone pushing that button instead of the bell? You'll end up hearing
more announcements because people simply don't pay attention.
As others have said, it makes an unpleasant environment. Companies such as Stagecoach request customers not to play videos or music aloud because it is a nuisance to other customers, so it is acknowleged that incessant noise is a nuisance.
The two things aren't comparable. Playing loud music is antisocial behaviour while next stop announcements are necessary to assist people who have additional accessibility requirements. They're also helpful for tourists and anyone who happens to be travelling on an unfamiliar route.
I've previously stated that I'm not a big fan of them either - I find the announcements to be too loud but I understand why they're there. If it means that someone can travel when they wouldn't otherwise be able to do so, it can only be a good thing.