For those who think GT7 isn't possible at launch, remember that GT Sport was delayed a full year from its intended holiday 2016 launch window. I personally believe the game was done at that point but they spent the next year polishing the visuals, performance, and making sure the online was ready for a full launch.
It sounds a little extreme but I remember the devs saying they wanted to make sure the experience matched the expectations, so they probably used that extra year to make sure there were no graphical hitches and the framerate wasn't juddery. After all, GT5 didn't make the best impression when it was the first real GT game of the PS3 era.
I really believe the next GT had already started development by the time GT Sport launched and they were supporting the game with new content as other members of the studio worked on the next game.