I agree with most of your post, but I just feel at the worst bringing in another midfielder should be at the expense of Giggs/Scholes/Park. If Cleverley and Anderson stay fit then they will play, if not then we will have someone capable of taking over their responsibilities who isn't 37/38. I think this season would be the perfect time to slowly phase them out, rather than rely on them this year and then completely get rid next. You said yourself that when Giggs/Scholes leave we'll need to bring someone else in anyway so why not just do it now?
I don't think it's viable to do it now because of squad numbers and the fact we have already given Scholes and Giggs contracts. Scholes has already said that he retired because he didn't want to play 1 in 3. Say we sign someone to play alongside Carrick in 40+ games, where does Scholes fit in let alone a fit Cleverley or Anderson?