Same here. I saw them at the 'intimate' New Cross Inn and it was an absolutely magnificent night. So Thijs can't hit the high notes anymore - he's in his 70s, that's not surprising. But his organ and flute playing are as good as they ever were and the band are furiously good.
I've always thought a band/artist should go on for as long as they want to - who are we to begrudge them their right to play their music?
That said, the current incarnation of Yes (No) has forced me to reconsider my position. They are offensively bad, to the point where they simply aren't capable of playing the music - Geoffrey Downes, I'm looking at you.