You guys have really covered a lot of things I've been thinking on this subject, people associate the holding role purely as a defensive one, but that safety net aspect helps the other two midfielders to play higher up if they know they can trust the holder to cover the ground behind him, then if the holder is also an excellent passer who can thread balls through the lines, it means it becomes easier to mount consistent pressure on teams, look at when we had Carrick and Scholes, we smothered teams with the way those two kept slicing through the lines into the forwards.
When it comes to the Am situation, if we get Sancho then I think he can slot in there is Bruno is out, or he can play LW if Rashford is out and Greenwood comes in on the RW, that's a pretty strong set of options within the front 4, I guess the question mark is if Pogba is the one out, but then if the holder we get is a good passer than a box to box like Fred or McTominay can be more attack minded in a midfield 3.