Re: What to do - Telling sensei their waza isn't good any more
Sometime physical skills deteriorate, for various reasons. Much more common is stagnation - probably more common than most people would like to admit.
It's very difficult for many student to think of their teacher critically, but I don't think it's wrong. Actually, one of the most difficult parts of teaching in this kind of traditional set-up is nobody tells you when you're full of it.
Even if the teacher is perfectly fine there often comes a time when it is time to go - there's nothing wrong with that.