When factoring GT:S vs Forza Horizon 3, based on the data available, it wasn't a shutout. In fact, they sold about the same (give or take - which actually looks much worse on GT). They lost their footing with Sport and should, in my view, forget about it.
We don’t have exact numbers, but including digital GTS is likely ahead of GT6 already. Imho GTS was always meant as something to perfect the online base and bridge the time to the next numbered release.
The biggest mistake was GT6. They released it too late when they should’ve started working on a PS4 game instead. The way MS did it. That’s the main reason they’re still behind on content for a full GT7.