That being said, it'd be immensely galling if we lost him on a free yet again.
Well, that ain't happening unless he wants to stick around for the time being. There's no way he'll be sulking about for the remainder of his current contract.
I agree with what you say about marketability. That's a genuine factor. Pogba is the third most marketable player in the world, apparently. That should be - and indeed
is, objectively speaking - worth something. Nobody can expect to just move in and snatch him up at a relative pittance. This is all the more true because of at least one (significant) factor that has nothing to do with marketability: his relationship with United, as represented by the current manager, is by all accounts pretty good. Vast difference from the relationship between Pogba and Jose.
Anyway, just as a side note: the currently most valuable central midfielders (designated as such) in the world by Transfermarkt are Kanté and Pogba, both valued at €100.00m. If you go by all categories of "midfielders", the pair of them are second behind De Bruyne *.
* De Bruyne's primary position is classed as "attacking midfielder" whereas Pogba's is "central midfielder", which is debatable in itself - but there you go.