For France, from the RWC squad, Bonnaire, Servat, Nallet, Heymans and Traille are the oldies. Yachvili, Harinordoquy and Rougerie are unlikely to make it in four years, but who knows!
Servat will be extremely hard to replace. Kayser, from Clermont, has been playing well, so let's hope he will prove to be of international standard. Plus, calendar boy is still there.
We have a few backrowers to replace Bonnaire and Harinordoquy, I wouldn't worry about that too much. With time, I think people like Gallan (Toulouse) and Bardy (Clermont) will step up, players like Picamoles and Lakafia will continue to develop and others like Nyanga and Ouedraogo will do a glorious comeback (they've been outstanding since last season).
For Nallet, who is 34 but is showing to be up to the task for 2012 6N, and for Papé and Pierre (who are both 30, so may be around for long yet), the obvious prospect is Maestri, plus others such as Fakaté. Millo Chluski is still young.
In the halves, we will have Parra for many years to come, and Yachvili is only 30. Doussain can only get better and there is some hope that Michalak will end his career on a high. Beauxis is expected to be back for the 6N as well.
Centres, replacing Jauzion will never be easy, but Fofana is showing lots of class, plus Mermoz and players like David, Fritz and Marty have it covered until an obvious 13 appears. Roumours say that Poitrenaud is coming back for the 6N, so he could play 13 too. He has been playing there for Toulouse, and is a better centre than fullback imo. (Plus Bstareaud, who could also cover as a frtonrower)
In the outside backs, We are still waiting for Malzieu and Médard to deliver (the latter one has had some highs, notably in the 6N). Buttin from Clermont is having an outstanding season, and Palisson is still young.
So I think that France benefits from having no "irreplaceable player", except perhaps Dusautoir. We would struggle if missing Parra and Yachvili at the same time, but they're good replacements for each other. Clerc is a match winner, and should be around for some time, but we have done all right without him. As I said, the hardest to replace will be Servat, but let's hope he can hang around some more time.