Having versatile players is the solution. This dramatically increases the chance of playing time for squad players.
They have a choice. If they are unhappy we will not force them to stay with us.
Well, that's why we bought Umtiti, Gomes, Denis etc. Digne could maybe play RB if needed, who knows? Besides, Alba is phrone to injuries, and combined with general rotation I don't think it'll be a problem. At least not for some seasons (ter Stegen and Bravo are both still here).
I'm merely saying that while they can be unhappy, they are still under contract, thus meaning Barca can keep them if they want and if they feel they are needed. Which why players like ter Stegen and Bravo can whine all they want, but that's still not gonna make Barca sell them during the winter window like some seem to believe. Barca hold all the power.