I think GT lost it's audience to Forza. The forza series has regular upgrades and keeps a satisfying level of quality while alternating with the more arcade sub-series. This built the brand, even though i think they overdid it.
But polyphony is the exact opposite. They are histically slow at developing GT entries and that has been diluting the brand. GT6 is an amazing game, but it was too late. GT sport is not the sequal fans want and also got delayed.
Sony built a big brand with GT but it's letting it slip. They have to bet big on it and instead of laying off evolution studios, they should have made them work on an Horizon version of GT rather than calling it driv club and positioning it as a competitor of GT.
Quite honestly, as much as i like Yoshida, he is not doing a very good job in this regard. Sony needs to get serious about GT or they will be wasting alot of potencial revenue. If it isn't too late already. Myriad of poor decisions i must say.