GTAO is GTA single players. They both share the same world but with different (kinda connected from what I've understood) stories and new missions, events that share the play of 16/32 people.
Like someone said above, people would have played 5 months single player and some multiplayer.. some more, some less. R* created two games, the normal single player and the single player online (if that makes sense to you). They changed strategies and what's wrong in taking risks, rather than having the same thing, they are trying to innovate.
And vice versa. Most people out there are all about online gaming now (not me, I prefer SP). But if R* focused more on SP, than MP would be as bad at SP is right now, and more people would be complaining about that.
So in R*'s eyes, they made the right move.... even though the core of older gamers are upset about the amount of content left out of SP.
GTAs are supposed to be single-player games, multiplayer should be just a bonus. Like in GTA IV.
How you mean is supposed? Just because that's what happened in the history of GTA doesn't mean they can't try new things.