I think the bolded is disconnect between the perception fans have of obtaining world class players and reality. There is a tendency to think world class players are obvious and simply buying them ensures success. Look at Liverpool's front three. It's safe to say nobody thought they would be world class players when they were bought, but with the right system/coach and complimentary players around them look at them now. Liverpool sold their only world class midfielder (Coutinho) and yet they are still challenging without any world class center mids. Look at Spurs, Harry Kane and Alli came through the youth system, Vertonghen and Alderweireld were very good business, and Eriksen cost less than the likes of Lamela, Solado, and Paulinho. On the flipside, so-called world class players who went for big money like Alexis Sanchez and Coutinho aren't guaranteed to stay at that level in a new team.
That why I said if those teams I mentioned spend money wisely. World class players aren't some known quality and with teams like City and PSG even the bigger clubs have to be shrewder in the transfer market. United fans should understand that now.
In my opinion, a lot of this comes down to buy players that fit a system rather than buying players based on name or potential. For instance, I don't think Wan Bissaka is a good buy for United. Not b/c he isn't good, but mainly b/c United would be better suited playing more attacking fullbacks (I think Dalot can be that player) to take the pressue off the midfield to create chances (this is my opinion and of course I could be wrong). To me, that is main difference between City and Liverpool. It's obvious over the last few years they have bought for need, and when then don't get the player they want, they don't make panic buys for ill fitting players, they try work with what they have.
Your assessment is correct if United are only targeting, known world class players, but that is an expensive and, IMO, futile proposition. Recruitment has to be about fit, current skill level and potential (if they are young). Simply buying players the public considers world class might sell shirts but it doesn't ensure success on the pitch.