Grant is more of a coach than a player at this point. Yes, a younger goalkeeper could take his place in matchday squads, but it's arguably more beneficial for any youth goalkeeper good enough to make a matchday squad to go out on loan and play regularly rather than sit on the bench for most of the season if even that.
Heaton is a solid goalkeeper at the end of his career, coming off of a potentially career-ending injury. At this stage of the game for him, going back to United and having a punt at becoming the number one must have been too good an opportunity to turn down, and one that won't come along again. At the very worst, United get a good backup goalkeeper out of it, who can step in in the event of an injury/suspension crisis (i.e. both De Gea and Henderson unavailable). At best, he somehow establishes himself as the first choice.
With Henderson, you've got a young goalkeeper (in goalkeeping years) who has shown some potential, however, there's no guarantee that he'll kick on and develop into a truly top goalkeeper who can sustain that level for years (see Joe Hart, Paul Robinson, Ben Foster, Robert Green, etc.). With De Gea, you've got someone who should be entering his prime years, yet for some reason, his form has dipped dramatically over the past few seasons from the incredibly high level it once was. Who is to say that he won't regain that form? If he was sold to another club and he got back to that level, while Henderson stayed and didn't live up to the potential, you'd be kicking yourself. Conversely, if De Gea stays and Henderson gets sold and turns into a world-class goalkeeper, again, you'd be kicking yourself.
Nobody has a crystal ball, sadly. I think De Gea and Henderson will get preseason to work their nuts off and prove they should be the starter, and whoever starts the season on the bench will end up going on loan somewhere before the transfer window shuts.