Retiring at sea certainly has a nice ring to it. One would have to be in reasonably good health, though. HAL has had a few. I met one, known as Mama Lou, onboard the Zaandam a few years ago. She was a very charming lady, and told a poignant story of how she ended up there. I believe she'd be onboard for abour 4 years at that time (except for drydock times.)
I think "wattering down" ports and such could save money, but visiting ports is a big part of the cruise discovery experience. That being said, I'd imagine those who live onboard really don't care after a while. They've "been there, done that" enough that ports wouldn't matter.