It is more to do with perception than ability. If a band still has appeal and people will come and see them, what is the problem? It is not about how well a band can play but how well their followers think they play. Most people are not stupid and realise that Paul McCartney, for instance, is not as good as he was in his Beatles or Wings days, but if they are a fan they will be willing to overlook his shortcomings, in return for being able to attend a concert and be transported back to a time they loved. In short, as long as the fans want to see them, they should continue.