I think the biggest reason/issue why most of the PL clubs are struggling for such long periods is because they think all problems can be solved by splashing cash on known/expensive/PL proven players. Nobody really does any scouting/development to groom young players or buy talented players with potential that are less expensive. A big problem is the fanbases/management not giving clubs time to develop a sense of identity or playstyle. Everyone wants the prolems fixed by signing a couple players. United buying sancho for 120 mill is literally going to fix nothing and then the board will say they bought the big name so they can't spend at other positions.
You have to build the base before you start going for the big money signings. Those should be the ones to finish off a rebuild/team, not the other way around. Perfect example was pool building up their squad for years and then capping it off with the 2 big signings of VVD and Alisson.
Every PL club thirsting for Koulibaly at 80-100m makes no sense when you have guys like Konate and upamecano available and ready to be sold for 40-50m who can be pillars of your defense for many years to come. If the scouting gets better then you don't have to keep throwing away bags of cash on guys who are worth nowhere near what they go for (maguire, drinkwater, etc). I think what RM and bayern are doing is the best way to go about it. We're both snatching up all the cheap young talent and throwing them into our academies then developing them for the future. Then if a couple end up panning out you don't need to make big signings to fill the holes. Even if they don't end up being on the level you can still more than likely turn a profit or break even on most of them selling to low/mid table clubs and use the profits to buy other proven players if needed.