What's hurting SP games is multiplayer (if SP is indeed being hurt).
The problem with MP focused games is that they have an expiration date. Once people stop playing or servers shut down, it's gone. Meanwhile, a SP game will live forever. And you can play SP games on 360 without paying for Gold.
I game on 360 predominantly but still don't play online all that much. SP games are my personal escapism, like reading a book. And yet I don't have any shortage of SP games.
Shoehorning MP into every damn game, that irritates me. But whatever.
For the record, I am fine with used game sales. Just like I am fine with used cars, books, movies, clothes, everything. In fact, I'll buy more games new just because I know I can trade them in for a decent margin. I don't buy the industry crying about them. Just another scapegoat. Other industries do just freakin' fine. Games are darn expensive, people are going to buy used to save some dough.