What you say about age in tennis years is possible. But imo it all comes back to nadal's interest in the no. 1. Federer has talked about wanting the no. 1 back this year but if the back starts hampering him that's no longer feasible. If this happens I guarantee you nadal will also cut back his schedule because at that point he will not need ~ 6500 points or whatever the new points are to win the no. 1.
You bring up a good point about the other guys but Imo murray is not the threat that federer was because murray is not as good on clay and because murray overall is not as good as federer. Murray will lose more often to djokovic, cilic, del potro, tsonga, on hardcourts than federer ever did to his competition. Djokovic is more of an allsurface player than murray imo, but his endurance doesn't look great.
Basically the bar for no. 1 falls and rises with federer and nadal will adjust accordingly. If federer is on the downward trajectory and can no longer chase the no. 1 because of his age, this will bring some balance back to nadal's health.