I wonder how it is for other peoples perspective when it comes to GTA V, I am not talking about Online..
Online is a very valid part of GTA V, so I don't see why it doesn't count. It certainly counted when it was IV... and SA and VC through the use of mods, for that matter... without mods or online play, those games would have died already - but SA's still going strong, mostly thanks to SA-MP, MTA and other mods. Otherwise, once you've hit 100%, there's really not much left in the game. You can runabout, but is that ever going to be entertaining for more than months? It certainly wouldn't if you played III/VC/SA/IV for that long so consistently!
Basically, if you don't like mods or online play, get used to games becoming old as soon as you commit way too much time to them! Personally, I rarely, like never, even play games to 100% - perhaps it's just better to, once finished with the story, not TRY to get 100%, but just have fun and do whatever you run into until 100%.