It's a heady mixture of the wisdom of hindsight and ignoring the harsh realities of modern football, and we are in danger of projecting things onto the situation in retrospect.
James Maddison at 21 yrs of age, has 105 games under his belt, roughly split 50:50 between the Championship & L1. He's scored 25 goals, the majority of those in the Championship last season. You could say that he is established. David Brooks at 21 yrs of age, has played 1146 minutes of Championship football (12 3/4 games), mainly as a sub, scoring 3 goals. You could say that he isn't established. He had the build of a 15 yr old, was coming back from GF and unless he was doing things that were so fucking subtle & special that i couldn't comprehend them, when all things are considered £12m was a fair price for him. If we have a meaty sell on clause, even more so. I find it odd that blades can say with such certainty 'I knew he was special & we've undersold him'. Scout for the club, because you've got a rare talent. If you can easily spot a £20m+ talent after only 1146 minutes, then you should be identifying players for united to buy from the lower leagues.
As soon as he started turning out for the England u20's, Wales u21's and full side, and we didn't go up last year, his days were numbered with us. That's the reality of football. You can mention Grealish, but that's a different kettle of fish. Grealish is a Villa supporter, he's played premiership level football already, he's at Villa who have a good chance of promotion, Villa are a bigger club than us, and I suspect that he is on circa £20k a week as it is. When Brooks changed his Agent, his intention was clear, he wanted to leave. The players that DB were mixing with, probably had watches that cost more than what DB was earning in 6 months. Your head gets turned. The same that has/is/will occur at United. We pay £x a week, another club lets it be known that they'll pay 4 x £x a week. And what you get then is a player whose head isn't right.
Further add into the mix that we lose £5m a season purely by existing and the fact that our two owners are at logger heads, then someone offers us what seems to be a fair sum, then we'll accept it.