The fitness levels players are at now, it's ridiculous to just pull the plug on a player who's still performing...just because he could "go downhill".
United allways eyed up an improved replacement, they never willfully sold one of their better players because they hit 30. Fergie brought Scholes back at 36 cos there was no one better than him to add quality.
The Digne situation is completely different as it's a fallout with the manager rather than it being a strategic move to get rid of an aging asset at a high value. No fallout, he's still playing cos he's a good player.