That's a serious question. We've all heard singers who were still on stage years after their voices had become threadbare; we also all know of singers who retired, tantalizingly, when they were in prime form, apparently with years of good work ahead of them.
I'm raising this question because of a concert I attended by a veteran ensemble where it quickly became evident that one member of this ensemble was having serious, and all-too-obvious, technical problems. The player couldn't play in tune in high and medium-high positions, which ruined one movement of one piece the group played. The player had bow-arm problems as well; there were all sorts of extraneous sounds and problems with articulating the music as written. The bow was not under good control at all.
This player is a member of an ensemble; the other members must have noticed these issues, and after a look around, reviewers are noticing exactly the problems I heard. (The reviews I looked at noticed problems with a different member of the ensemble as well.) So....how does one go about conveying to a player that it's time? that the technical issues are dragging down the entire ensemble? What do you do when it's more than one member of the group? Who raises the issues? Other players in the group? The group's manager?
Do the ensemble's written agreements include a process for dealing with this kind of problem? In a perfect world, the contracts would cover these potential issues when an ensemble is new and everybody is in top form. If reviewers are noticing the problems and nothing is happening, it's a problem for the whole group.