First of all, USC is a private school. I don't know what their tuition is, but its a bit tougher to pay your own way there than a public school. Quality D1 talent will not likely turn down scholarship offers from good programs to walk on at USC, unless their parents are pretty wealthy (or if they have access to Reggie Bush's former agent).
A quick look at USC's 2010 roster lists 23 Seniors, and an amazing 32 Juniors. I could be off by a couple since my eyes are old, and some of those guys might be walkons. I guess they could redshirt some of those Juniors, but you are looking at about 50 scholarship players who will leave over the next two years. They can only add 15 scholarship players per year, for 30 replacements. They will lose another 20+ in the 3rd year, with another limit of 15 new grants in aid, leaving them with less than 60 total scholarship players.
I don't care how talented those 45 new players are, competing with less than 60 scholarship athletes is tough. Even if they do manage to get some 4 star players to walk on, it will not overcome their depth issues. 1 or 2 key injuries an they will really be in trouble.
Now, I'm not talking armegeddon here. After all, they still have Orgeron, the greatest recruiter in the history of college football. And we all know how awesome Lane Kiffin is, right? Nevertheless, once the 2 year bowl ban is over, USC is looking at another 3 years minimum to really be relevant again.....and that's if Lane Kiffin is truly better than Nick Saban.