When Is It Time for a Band to Retire?

I've been thinking about retirement a lot lately. I just turned 31 and with each passing birthday, I reflect on my past and future. My husband and I have some very aggressive, concrete goals for our future. Here's a hint: we don't plan on working until we're 80! In fact, we'd like to be done "working for the man" by 50 and if I can swing 45, great. This will take a lot of discipline on our part, but we're giving it a shot. All that said, I wonder if musicians ever really create a retirement plan?
This thought comes into my mind after watching the slow death of one of my favorite bands. Great White is a group of great musicians but singer Jack Russell is very ill and he just can't perform at the same level anymore. Lately, the band has been using guest vocalists to fulfill contract duties. Why not cancel the dates and reschedule when Jack is better? Obviously that's a short term solution - but what about the long-term?
The sad fact is that our favorites won't be able to tour endlessly for the next 20 or 30 years. At what point do bands hang it up? That's a hard thought for someone like me to handle. I find it comforting to know that Poison will be out on the road next year. That another Crue Fest is in the works. That Van Halen will likely tour again soon. But these bands are on a different level than the likes of Great White or Dokken.
How does a band like Great White bow out? There's a fan base there for sure. There's also the lasting stigma of the Station Nightclub fire. There's "Once Bitten, Twice Shy" and then there's painful YouTube clips of Jack clearly in agony at Rocklahoma '09, trying his hardest to perform. He gets my respect for fulfilling his duties and doing his job but it can't go on. At some point, it's degrading the legacy of the music.
Or maybe I'm wrong.
Maybe the band can keep on as they are, grabbing guest vocalists and waiting for Jack to heal. I really don't think so, though. I have to believe the other guys in the band want to retire like "normal" people someday. Sure, musicians choose a different path and generally shudder at desk jobs...but at some point, even performing for crowds of people night after night becomes work and the road wears even the most grisled guy down. Rest is rest.
Do I think Great White will hang it up anytime soon? No. But I do think we need to brace ourselves for a long list of retirements within the next five to ten years. As it should be.