Money hasn't been an issue for Macca since 1963. He plays his songs to audiences because he is a musician/song writer/performer, and that's what they do. While he can fill 90 stadiums on a tour, I guess he's right to continue. Good luck to him.
I also heard that he's sticking to the written key, which does seem a little foolhardy at his age and apparently doesn't warm up before a show either. The only other singer I've heard of who is still singing in the original key is Stevie Wonder. I did read that SW has weekly singing lessons/workouts, to keep his voice in shape.