I have several hundred hours into gt sport. While it doesn't have the single player features of the mainline titles I could care less. I personally find no fun in racing ai controlled cars, especially in Gran Turismo games where the ai is garbage.
The thing about gt sport is that the racing is top notch. Sure there's a few undesirables floating around but for the most part it's super clean and respectful. No video game or genre gets my heart pumping more than door to door racing against real people.
On top of that, the sport mode races offer a balance of performance system which works fairly well, so there's often a great mix of car models in any one race. Another reason I like the BOP system is tuning, and not in the way you might think- tuning is off in sport mode. For me, I don't have the time or patience to test and tune every car for every race, so the fact that everyone drives on the same tune is awesome. I never have to worry about the guy who just blew my doors off having a glitched tune, he's just faster.
GTS has a few pretty minor issues but overall, it is the best place to race on console, imo.