I said it outsells other games, I said nothing about the other games aren't selling well. But Rockstar outsells them. Why? making their game more accessible to the majority, versus catering to the minority of players. So your argument you made up for me, is not something I said or even implied.
Right, because all other games that could be considered action shooters don't sell well and don't cater to the majority. That's a ridiculous premise (not to mention you don't have any sales data to back you up).
Also note I called it an action/shooter as I realize the game isn't just a shooter. It's like arguing You could argue its an action sandbox if you want (side note: arguing about a game's genre using a mini-game as an example is absurd; core gameplay features define the game's genre). COD isn't a close match but you were the only one to mention COD. If you want to compare similar games better examples are Sleeping Dogs, Saints Row, Crackdown, and Red Faction: Guerrilla. Does GTA outsell those? Yes. Is it because the other games cater to niches? No, it's because GTA is the most polished (as it should be because it was first).
As far as action rpg, considering series like Diabo and Torchlight are the prime examples of action rpgs, going have to call that classification off base.
Whether you feel the classification is off base is moot...It isn't strictly a shooter...and the games that come close to being what GTA as I said their sale numbers pale in comparison because of Rockstar catering to the casual player.