I take it you never watched Schmeichel play for United ? An alarming percentage of the goals he actually conceded were horrendous errors. Each season he would have a spell of 2 or 3 games in which any shot on target would find the net. Yet he was probably the best keeper United have ever had due to the incredible saves he made and the extent to which he frightened the opposition. I rate him as the 3rd best United player I've ever seen, behind Ronaldo and McGrath.
Goal-keeping errors are a part of the game. You hope your keeper won't make many but until you've seen a keeper in action for a season or two, 50+ games, you really don't know how their error-rate compares to others.
There's a big difference between making an error then playing ok, and making an error then going to pieces (Taibi, Jim Leighton for United, though not for Aberdeen)
Romero is a competent keeper; who knows whether he is De Gea standard if given 2 full seasons at United. He would need to have improved since his Sampdoria days but that's several years ago now so not unreasonable to think he might be a better player. His experience of playing for Argentina strikes me as being a close approximation to the pressure of playing for United.
I rather suspect United would concede a similar number of goals regardless of whether De Gea or Romero is our keeper.
The difference is, more often than not, Schmeichel would save us points, let's not forget him in 98/99 either, the fact you're comparing Romero to one of our all time greats is beyond me but that's fine, I'm not going to argue who makes more mistakes between a second string goalkeeper and one of the greatest in our history.
Yes goal-keeping errors are part of the game, De Gea, definitely, unless you don't watch him, way more often than not DOESN'T make mistakes, very consistent and pulls saves out his backside to save us on a regular basis, add that to his aerial presence and you have something, whereas Romero, not sure if he can hold the ball or if he'll drop it like he did a couple times under LVG.
Like I said, I know Romero is competent, but he's just a quick fix, forget his age, because we also got VDS when he was older so that isn't an issue, the issue is, Romero has the potential to drop us points, De Gea gets us points, I don't know about you but I want a keeper good enough to gain us points, a world class keeper. There's a reason no bigger clubs are sniffing round Romero, look at Arsenal, Cech who isn't great at the moment, why don't they go for Romero? Because he isn't good enough for them, if Lloris leaves spurs, would they? Doubt it. City? Nope they don't want Romero. I certainly don't feel like I'm crazy, i've seen Romero put in hugely decent performances, I'd say he is more than a sub, less than a starter, can play a decent role at a club like Southampton. Now let me ask you, do you want to be like Southampton who we just beat in the League cup final, or do you want to be United?
I know you're going to turn around and say, we need to see Romero in a run of games to truly judge how good or how well he can do for United. The issue with that is, do you really want to replace arguably the best goalkeeper in the world with a 29 year old that has never made it big even whilst being at the club for a year and a half? There's a reason why most fans don't want him to start, there's also a reason why top 4 teams are not actually going for him, Liverpool, Arsenal and City don't have capable goalkeepers and we've seen that time and time again, if he really was good enough I'm sure they'd have a sniff, infact he isn't even being linked by anyone reputable, if our fax machine had worked, maybe we would've found out if he was good enough, but from what I've seen, he isn't starting United quality.