It is not about replacing, it is about enforcing.
When Giggs, and Scholes indeed do retire, we will be a lot weaker. However, this season I think despite spending only £7m, that we might have enforced our forwards immensely, both with a very successful loan deal with Sunderland with Danny Welbeck, and the brilliant signing of Chicharito whom both might end up world beaters. And hopefully Macheda might turn into that too, I hope his Sampdoria loan deal will last for 18 months rather than just thus season. We now have a bunch of forwards that are good to go for years. We have plenty of midfielders with a long time left, Carrick, Anderson (he can still develop a lot further), Fletcher, and Cleverley has been brilliant at Wigan, he will return next season and become a squad player of value. Even when Giggs and Scholes do retire our midfield coverage is pretty strong in midfield:
Fletcher, Anderson, Carrick, Gibson (central midfielders)
Nani, Valencia, Obertan (wingers)
Park, Cleverley (any midfield position)
We also have some exiting academy arrivals in the likes of Ravel Morrison and Gyliano van Veltzen (I hope Ajax became pissed for a reason, and he is said to be a left footed left winger and that is something we could need)
Now as is quite obvious both central midfielders and wingers can clearly be improved, Valencia and Nani are both great, but they are both best at the right. And as Nani, Park, maybe even Cleverley is a good option on the left, and Giggs and Scholes are good options as attacking midfielders. Whoever we bring in does not have an immense pressure to perform at once because we have a lot of quality with years to go. Same thing with whoever replaces van der Sar, if they should struggle adapting and need a bit of time, we have brought in a very talented goalkeeper in Lindegaard, and PIG is not useless neither. We have strong backup in any position we risk losing to retirement and that is a very good thing.
We have Fabio, Rafael, Evans, and Smalling behind our defenders too, very good backup and are braced for impact when they are not good enough anymore, but Nemanja, and as if he’s injury free I bet Rio has plenty of years left too. We are more braced for this generation change than any previous Generation swap. Both in goal, defense, midfield and forwards.
When we have lost key players in the past, Ferguson has always changed focus to others. When we lost Cantona, we relied on a immense midfield, perhaps the best ever assembled, in Keane, Scholes, Giggs and Beckham. When we lost Ronaldo, Ferguson built the team around Rooney, and that would work great if Rooney did not injure his ankle. This season with Rooney struggling he built it more around Berbatov, again a tremendous success. And even though it looked like Berba have a much better season this than in the past, I am not so sure. Even though he scores a lot more goals, you have to ask yourself where has all the assists gone? Same thing with Rooney this season, he does not score, but he has 10 assists in the Premier Leauge, something he had very few of last season.