Please do. I'll show that you are strawmanning the many reasonable objections that have been raised, including my own.
[If your comment is only in reference to anyone who believes that he will be an unmitigated failure, or that he is 'shit', 'average', etc, you should have been more clear. I wouldn't personally bump a thread with the specific intention of embarrassing someone, but only because it's pointless attempting to reason with anyone who believes that 'he's shit' is a worthwhile comment.]
There are hundreds of players around the world who could turn out to be a good signing for this club, but that is not a particularly compelling reason to support an individual signing by itself. I hope that this doesn't blow your mind, but I'd go even further than you have. I believe that Young would be a fantastic signing, but I would still prefer that the club didn't sign him. Contrary to what many clearly believe there is no contradiction between those two statements.
To underscore this point even further, John O'Shea has been a fantastic servant to United and a very good player. But it's not unreasonable to believe that there are better players in the world and that United need to improve in the areas where O'Shea mainly operates, which may include looking to replace him. I don't personally believe that, but it's not obviously wrong, either.
What we believe about an individual player or team depends on the context in which the question is presented. So, for example, if United had been relegated to League Two, it would not then be unreasonable to believe that most of the players in League One could be considered 'good signing'. Similarly, Young would probably be an even better signing for Liverpool than United, because they don't have any recognized wide players of that type and quality, and also because of their current position relative to several others in the league. In other words, it's not objectively true that Ashley Young would be a 'good signing' in all possible circumstances.
I have articulated my own objections starting on page 15 of this very thread, so I don't want to repeat them in full here. The basis of my concern is that I believe that there is the potential for genuine improvement, paticularly and most crucially within the first 11-15 players. Some of that will undoubtedly come from the players that are already at the club -- Hernandez, for example -- but the evidence of the season just ended persuades me that this current team is some way short of the standard that we have reached in previous years.
The performances, the results, and the style of football, were all dissappointing at various stages last season, and it would be a mistake to ignore all of that simply because the club enjoyed another hugely successful season. It's fair to say that we are some way behind Barcelona, and possibly even Real Madrid (although that's arguable), and there are also teams in the Premier League -- Chelsea and Man City being the most obvious -- who will be looking to improve, and have the money to attract players of the highest quality.
The issue that I have with signing Young actually has very little to do with his ability to perform exceptionally well in most circumstances. I'm just not convinced that he quite has the ability to elevate the team to a much higher level than it already occupies. He will almost certainly improve the squad and provide a very good option on left. And his ability from set pieces alone will be extremely valuable. But is that enough? Will he be chosen ahead of Nani, Valencia, or Park in the games where his qualities would be most valuable, for example? I'm not so sure.
I should state that this argument is entirely context dependent. If the club signs one or perhaps two other players that provide a genuine boost in quality, and if certain players in the first fifteen improve sufficiently, it's entirely possible that these objections will melt away.