GT6 could've expanded to PS4. I don't understand what they did to keep the game locked on PS3 system forever. The Last of Us was supposed to be the swan song of PS3 but Naughty Dog still ported the game and found bigger success on the PS4 than it did for former.
TLoU is narrative heavy. GT6 isn't.
I don't really see the connection here. Polyphony having to go back and port the entirety of GT6 is an undertaking that ND would have most likely not bothered to do.
I doubt the financial backing would have even been worth it.