Van Gaal didn't sign alot of old players though and when he did they were on the cheap (Valdez on free, Romero was on free). Bastian and Falcao were given huge salaries but they were on loan/bought on free. The rest were relatively young. Blind and Rojo were 24, Schneiderlin, Herrera and Darmian were 25, Di Maria was 26, Depay was 21, Martial was 19, Shaw was 18. LVG also believed in a small squad, maybe too small for his own good. He ended up relying heavily on youths one of whom was great ( Rashy) but most were absolutely dreadful (ex McNair). Mou started this madness as he spent money and gave ridiculous salaries on players who were at the wrong end of their career (Sanchez, Matic, Mkhitaryan, Ibra) and would have added more if given the opportunity (ex Perisic). Ole took it to the next level as he not only bought players who were at the wrong end of their career (Telles, Varane, Ronaldo, Heaton, Mcshane) but also gave contract extensions to players who shouldn't have been kept ( Matic, Mata, Grant etc)
In my opinion this is the result of managers given too much power on transfers. The manager's job had grown so much in modern football that can't be expected to know and decide over that talent playing in Portugal or Argentina. Our managers didn't help the cause either with new signings being frozen out of the team almost immediately and for no good reason. That started with Mou with Fred but precipitated greatly under Ole. He barely gave the signings made in his second summer transfer window a sniff of first team football.