Of course I understand that buyout clauses have negative consequences for clubs and I also fully understand that elite clubs are willing to pay higher wages in order to compensate players for not having one. However, I'm speaking of this particular case, not the general one.
For me, your arguments ultimately come down to pride and pride is generally speaking a bad advisor when doing business. Assuming that Haaland generally preferred you over Dortmund, the choice for you was between signing him with a buyout option or not signing him at all. You guys think "we're Manchester United. Barcelona and Madrid don't grant players buyout clauses and we are also a big club, so why should we?" when you should actually be asking "why can Madrid and Barcelona sign those players without clauses and we can't?"I know it's a bitter pill to swallow but at the very moment, you don't have this third option of paying higher wages and not grant buyout clauses at all anymore. You are all like "Dortmund is a selling club, we aren't" when the whole point of being a "selling club" is that you develop players of a quality you otherwise wouldn't get your hands on. And that's exactly the problem you're having: The elite isn't willing to sign for you any longer. If a player nowadays plans his career, you aren't his ultimate destination. You may be considered as an intermediate stop but not the club he wants to spend his prime years at. Acting like a top club doesn't automatically make you one. Imagine a random small club would just copy that behaviour and in negotiations with a highly promising youngster would say "no, we won't give you a buyout clause. Sign for us without one or sign for someone else." This would just come across as silly. You can only play that card when you actually have the position of strength necessary for it and United IMO lacks this international standing.
In essence, you are still acting like you're one of the absolute top clubs when you actually aren't any longer. My point is that instead of refusing reality you should accept your current position and think of a way to regain your previous status. And for that, Haaland would've been a great signing. You argue "we develop him for 2 years and then he signs for City and Liverpool? Nah." but you'd have sorted out your striker problems for 2.5 years with a guy who's already good enough to lead the line at a top club and guarantee you 20+ goals a season and if he eventually leaves then this sends a signal into the world that improves your reputation as a good place to develop for young players. And in the meantime of those 2.5 years you'd have time to scout a successor for Haaland without pressure. And if he really leaves for one of your biggest rivals, what's the problem? If he goes there after spending 1.5 or 2.5 years at Dortmund, you'll get the same result.
So in the end, all of your issues with this deal are of symbolic nature. I don't get this kind of thinking. Imagine for once that you applied the same strategic approach Dortmund is currently applying. Take a step back, build up a young, talented squad and sign players like Hakimi, Sancho, Haaland, Brandt, Dembele, etc. and don't block their development by having (or signing) experienced players in your squad that may be just a little bit better right now but ultimately have a much lower ceiling. Over 1-2 years, you'd assemble an extremely talented squad with a great promising player in every position, pretty much like Dortmund has now. The only difference is, you have the financial prowess Dortmund lacks. That means you have much better chances of actually retaining those players. Imagine Dortmund could go into negotiations with Sancho, offer twice his salary and say that the strategy for the upcoming years is to hold this squad together, spend big on another two or three top stars and ultimately challenge for big titles. You certainly could. Of course one or two of those players would still leave but that doesn't matter, a club can compensate that. Liverpool also lost Coutinho who seemed irreplaceable for quite a while. And they lost Sterling to City. That's part of the business, everyone has to go through it.
Thing is, you want to make the second step before the first. You want to avoid the uncomfortable part and you are doing that for far too long already, essentially making matters worse.