Except it's not a Xavi vs Allen comparison thread - he even said it in the OP. It's a - do we give a chance to one of the most promising talents in world football right now and watch him grow as a United player, or do we go for the finished article who's intimately involved with Barcelona, and will always be a Barcelona player, first and foremost. With the latter, you know you're getting an all-time great, a truly special player - but you're going to miss out on his journey as a developing player (which Barcelona fans got to witness, and remember now with great fondness).
Some don't believe in that romantic guff, but others do - and it's their prerogative. If Pexbo made a similar thread proposing a De Gea - Buffon exchange in 2011, the reactions would've been similar. A keeper who made a few howlers in his first season, vs arguably the greatest of all time who had already done the growing up stuff at Parma and Juventus. And in hindsight, De Gea has proven to be a really good option, and it's been a pleasure to watch him go from strength to strength.