I don't think it's about time or effort spent, though. Most of our thinking around Maguire is probably on avoiding a book loss because we are sailing close to the wind after this summer's business. His not very good is still our best 4th choice CB good so until we can achieve a deal that enables a replacement within FFP parameters then it's more about financial pragmatism and maintaining squad depth than pride over what we spent on him.
Let's say we sell Fred and McTominay for suitable fees, then maybe the "sunk cost fallacy" comes into play as we may be able to absorb a loss on Maguire simply to shift his wages and rather cumbersome presence as squad depth.
I do not think we will have much FFP problems if we sell McTominay, even if we sell Maguire with a small loss. Essentially, the contracts are amortized so while we would be spending around 200m (providing we buy Amrabat too), those players are on 5 years contract so effectively in the books that is 200m/5=40m. Just selling McTominay alone for 35m almost covers those signings. We also sold Elanga and Zidane for around 20m. We will probably sell Fred too, so we should be fine even if we take a 5m or so loss in Maguire. Bear in mind that we quite lowered the salary bill by releasing Ronaldo and De Gea, and we can do it further by selling Maguire, Fred, McTominay. So we should be ok with FFP.
Also, I do not think we need to sign another CB this season. Shaw can play there as fourth choice, in which case one of Dalot or Malacia deputizes Shaw as left back. We can play Evans there too for cup games, and maybe even Bailly who is essentially unsellable and in the last year of his contract. They are worse than Maguire, but you do not need that good players in the early stages of League Cup or FA Cup, or in matches where we are winning 3-0 or so and want to give a 20 minutes rest to Varane or Lisandro.
So all in all, if we get 25m for Maguire and save 20m on his salary (next 2 years), that is 45m saved. That is too much for a 4-5th choice CB, who won’t have any resale value.
Honestly, I think we should have sold both him and Lindelof and buy Kim. I think Lindelof is a quite good CB and good backup, but he is also 29 so his resale value is lowering. We might have generated enough money from them to pay Kim’s clause and then have 3 very good CBs. I think quality > quantity, we do not need to have too many players, as long as they are quality. For whatever reasons we try to have a giant squad but without much quality.