Red Dead Redemption wasn't on PC and it did fantastic. GTA V may not be on PC because if you look at GTA IV and LA Noire, you can see they do not know how to properly optimize. Adding anything else to what was GTA IV would make GTA V completely unplayable on 90% of computers.
The artist formerly known as Luft
Roc city social club?
Nope, they couldn't care less about how people mod their games. The actual quote was this
"All we can say is that whenever it is viable (technically, developmentally and business-wise) for us to release a game for PC (or any other particular platform) – we will and we usually do; unfortunately, that is just not the case 100% of the time for all platforms."
Pretty much, they can't optimize well enough to make anything more advanced than GTA IV playable.
The artist formerly known as Luft