I mean a year ago nobody was saying Chelsea had a great squad that was going to fairly comfortably win the CL and then with one new striker go on to mount a PL title campaign.
United have a really strong squad, it might not be as good as the 3 teams mentioned but it's a lot closer to them than it is to Arsenal or Tottenham.
I think if Pep went there, he'd have a lot of players he'd do well with:
De Gea is a bit of a loon but he's a great shot stopper on his day. Henderson is a guy I think Pep would like to work with.
Wan Bissaka would be a milder version of Walker with someone to teach him where to position himself, which Pep would do. Pep would love Shaw's attacking credentials and again teach him where to stand in defence. Maguire has proven to be a high quality defender at Leicester and for England. Maybe not spectacular but he's great at carrying the ball forwards and strong in the air which Pep would love. Varane would be good under Pep too.
Fred was great before United, and we weren't linked with him by accident. Van De Beek was a hot prospect from Ajax who'd have been primed to play the Pep game. Pogba has shown that in a structured side he's actually good, and Pep would love his ability to play long passes.
We know Pep rates Sancho. Cavani is a workhorse + great goalscorer. Fernandes might be overrated but under Pep his direct goal influence would be made use of. Greenwood the same as Fernandes. Rashford is like Shaw, a good manager would teach him to not be so stupid all the time. And Ronaldo knows where the net is, even if he is a lazy primadonna Pep would always have time for a goalscorer (if he is willing to drop the ego..).
They do have some dross that Pep would bin off, but they have a lot of players that you can see have suffered from terrible coaching; Rashford being the biggest victim.
We can pretend the players aren't great but they came 2nd last season with Ole at the helm. If the most poorly coached team in the league finishes 2nd then you can probably say that the players are good. This is why it's so important Ole stays.