Keeping him for the next season is another matter. But this season, unless he's mathematically out of top 4 race or has huge fall out with the squad, he's staying till the end. Might as well just accept that and move on.
And lets not pretend that even after the season replacing Van Gaal won't be tricky.
Mourinho is not an option, he plays completely different style and you'd have to start rebuilding from scratch. Simeone, same problem, you have no personnel to play in his prefered style.
Pep has succeeded making transition to posession-based tactics in Bayern, but lets not forget that he had massive, massive advantage in quality of players available. There's no telling if he can pull it of in United with Fellaini and Carrick in the pivot.
Ancelotti is locked for Bayern job by the looks of it.
Who else is there, Rafa?