Because of the mistakes in how he built the squad. Getting the most out of players counts when it's in a format that's sustainable for long term success. We bandaided problems for so long. When it comes to modern forwards Rashford would post nice stats but Ole's barebones tactics would reinforce his weaknesses ie teamwork, passing and anything team related. Now that Ole has gone it's become obvious he has nothing in his bag outside of running and shooting. Rashford never developed his team based skills and it will probably plague him for whatever might be left of his United career. Strikers being extremely greedy is a dinosaur concept from era's past. These days teams play front 3s not 442s with specialists.
With our midfield we played a deep lying counterattack that hid just how deficient our midfield were at supporting a high line. He thought he could just play one way for 2 years while rebuilding for a totally different style. If the goal was to play proactively it wasn't a very well built midfield in any shape or form. ETH now has to patch it asap.
Ole and Murtough just began to build the squad. Whether or not Ole was the manager going into this previous season or next season, the squad was always going to be building.
Outside of Marcus, whom I don't disagree with his limitations, CF was going to be addresses two fold even without Mason.
Central midfield was looked up last year but wasn't addressed due to Ronaldo and inability to shift other players. It was always going to be urgent knowing Pogba, Matic, and Mata were going to leave within 12 months or sooner.
So it isn't just all about Ole. The club has to have a vision of player identity and recruitment that is of a certain level, bar the manager. It's pretty clear it has changed a lot with things being more streamlined through Murtough and the collective scouts, academy personnel, etc.