It seems like with the players you mentioned your history does not go back too far but just to clarify with the players you mentioned;
Rooney was as close as a certainty to succeed in football that I have seen, Ronaldo had big big potential and De Gea was first choice at a very good Atletico as a teenager.
Odds on the three succeeding were pretty good if you know anything about football.
Our history is based on bringing young players through but not necessarily splashing big money on young players as we have a couple at the moment that still have to step up to the plate.
Rooney only had 15 goals to his name before he signed for United. He only really hit the headlines at Euro 2004. There were still plenty of people on this forum that thought £28m was too much and were scared he'd flop. That was basically the same as spending £50m+ in the current market.
We signed Ronaldo as a 17 year old for over £12m in 2003... That was huge money then! People were still calling him a 1 trick pony and saying we should flog him for what we can get until he really started firing in the 06/07 season.
DDG had only played two seasons of first team football at Atletico before we signed him to replace VdS, arguably one of our best GK ever. We paid a then British record fee for a GK. Atletico also weren't that good then, think they finished about 7th in his first full season.
The reason they had a greater chance of succeeding than Højlund was the fact we were the best team in England then and SAF was our manager. If they were joining tomorrow, I don't think it's crazy to think that they'd have just as good a chance of fulfilling their potential than Højlund has of fulfilling his here.
Also, I tried to pick recent examples, but we can go back further. We signed a 23 year old Roy Keane for a then record transfer fee, even though there was no guarantee he'd be an immediate starter with Ince and Robson already at the club. We signed him because of the potential he'd shown.
We've spent plenty of money on potential, none of it was guaranteed returns, so I don't see why you're being such a misery guts when we're about to do it again on this Danish lad.