It's basically this.
IMO, with Ole, the team will always have a maximum level that is the sum of its parts. The only way for Ole to improve results-wise is therefore to increase the sum of the parts by buying externally, because the parts at his disposal don't seem to be improving much. The improved players under Ole can be counted on one hand, one of which is a player who we all knew was talented and who had had great seasons before, but who was frozen out under the previous manager (Martial), and the other is a player who was always breaking into the first team based on youth team showings (Greenwood). Rashford looked to be improving and was on track for a great season, but then he was over-played and exacerbated an injury which set him back quite a lot. Fred was looking very good but is out in the cold now for some reason.