Honestly if we don't sign him, it will just be totally detrimental to purchases of future world class stars. I worry about us without Ferguson. He unquestionably was a huge reason players signed for us over City, Chelsea etc. What's to stop them now?
Why would this transfer matter so much considering, as you said;
Barcelona are in top 3-4 clubs in world footballs history. Cesc Fabregas is a Barcelona born player, who always wanted to play for his local team. With Thiago gone and Xavi aging they aren't going to sell him easily. Dortmund agreed to sell Gotze to their rivals Bayern. If you think they wouldn't sell Gundogan for 40-50 you'd be mad, they'd snap our hands off.
Missing out on a player in that situation is hardly detrimental to our future.