You know this same rubbish was said when PC gamers were waiting for GTA 4. I clearly remember people posting "well you didn't get Red Dead Revolver on the PC so you won't get GTA 4 either". The two franchises are completely different and made by completely different studios.
It would be beyond stupid for Rockstar to refuse to bring GTA 5 to the PC. They already have the game engine "working" on the PC it would require very little work and financial outlay to get a PC version out the door and unlock a further 2 - 3 million sales.
Then couple that with the fact any PC version they make will be "easily" portable to the Next Gen consoles when they release it just makes sense for there to be a GTA 5 on the PC sooner or later.
Guess we will see about that. I'm not saying it's unlikely I'm just saying it wouldn't shake my world to see them not do it.