Honestly I just think that Shaw is pretty good and not as bad going forward as people seem to have decided, and that Williams is already a decent back-up and might well turn into a really great fullback. I don't believe in spending money on positions where you are good. Teams get better when they have stability over a number of seasons. Whatever degree of improvement on Shaw we might be able to buy would probably not balance out the difficulties created by the risk that this other player just doesn't work out, that we have to spend a season settling them in, that they do ok but adding a load of new names disrupts the squad all over again. There are positions where we can so obviously and dramatically improve that a signing is a good idea, like RW and DM. But LB? You'll do more harm than good.-I don't think we can contend for the league and Europe and have 2 fullbacks who aren't above average going forward, and Wan-Bissaka is more reliable than Shaw all in all.
-Williams did fine but he wasn't exactly Evra out there and honestly that's what we need. I think he'll be a useful squad player because he can play both sides, has good energy and aggression going forward and is a clever short passer for a fullback
-Dalot does have the potential one would think to become a proper threat, but only on the right
If you wanna argue we should just spend big on the next Brazilian teenage LB who projects as Marcelo going forward because Shaw and Williams can handle the spot for a couple years then fine, but Liverpool and before that Real Madrid had elite fullback creativity when they won the CLs (Robertson/TAA and before that Marcelo and even Carvajal) and we have very little at the moment.
And if we want to play 3 at the back more where Shaw did well as an LCB, we need that proper assist machine at LWB, be it a left back who is great going forward (Grimaldo? Max?) or a left midfielder who can play there like McNeil from Burnley or Saka from Arsenal (contract ends in a year) and the other names that come to mind aren't realistic transfers at the moment (Sessegnon).
I haven't seen much of him but it seems like we might have missed out by not buying that Skov guy who scored a million goals, since he did that cutting in on the right as a winger but has apparently done well at LB and LWB in the Bundesliga and could have been our 3-5-2 LWB and our backup RW, hopefully to Sancho, since James is better on the left or might even have to play on the right since we might not buy a proper backup for Bruno and have to make Sancho our backup 10 or else have a midfield with no creativity (assuming Pogba goes). Might have been 10M well spent. Mourinho wanted a player of this profile and thought it could be Perisic, I recall.
I also think our fans always, always want us to replicate what the current best teams in the world have just done. But those 'best teams' tend to get there by doing something new, which tends to be a case of their quality managers responding to the resources available. When Barca were killing everyone with tiki taka, we were all clamouring for delicate Spanish possession-favouring midfielders. Then Dortmund (and to an extent Bayern) shook things up with more direct, less intricate styles of attacking football and all of a sudden we all hated LVG because he prioritised possession football, forgetting that in Fergie's last years that was what we all wanted.
Currently, a lot of good teams have very attacking fullbacks. A lot of their creativity comes from those fullbacks, whose defensive weakness they cover with strong, hardworking DMs. We don't have that. What we do have is probably the two best defensive fullbacks in the league. One of them is one of the best defensive fullbacks in the world right now. Rather than jumping on Football Manager to pick out some marauding wingbacks to expensively replace them, why not actually play in a way that suits our assets. Stick Shaw and AWB in the back line and, secure in the knowledge that they won't need much help from their midfielders, go with a Bruno-Pogba-Fred midfield which is defensively light but will provide loads of ammunition to our forwards.
We should try and be good at being Manchester United, building on our best Manchester United players, instead of spending loads of money to replace those perfectly good players with a different type of player just because that's currently what Real Madrid and Liverpool are doing. By the time we manage to make that happen - two seasons of trial and error and bad form and disruption - the European giants of the moment will probably be doing something completely different.