It's true that R.E.M.'s legacy would be that much greater if they'd broken up when Bill Berry left (I still think New Adventures in 1996 was almost their best album), but the good thing about leaving it alone now is that their legacy will grow over time. People will come to re-appreciate them, and some of those post-Berry records like Up and Reveal that are actually pretty good will eventually get re-appraisal.
Kind of a weird non-feeling about their calling it quits. I should be somewhat sad or surprised, but I'm not. I think, spiritually, I feel like R.E.M. have been gone since the college/indie/alternative rock scene passed its peak in the mid-90s. Anyway, good luck to them. They were an amazing combo, and classy to the end.