You have racing against real people and AI races now in GT Sport, if you don't use the latter, and have no interest in it, then you could ignore that in a new GT game too.
A new game could offer more cars and tracks, which would benefit the online and offline racers, and of course some people like both.
No interest in uprating/tuning cars! Then don't do it. No interest in creating car/suit/helmet liveries! Then don't do it.
GT Sport has catered mainly for the online racer at the expense of the traditional GT format players, (whatever aspects they enjoyed
) but with the power that a new console could give, all aspects of the previous games have the possibility to be enhanced, the question will be what aspects of previous games will be in the next game.
I bought a PS4 for GT Sport, and for me the game should not have been the price of a normal GT game for the content contained. Add to that that there is very little game without and internet connection, and the value is even less, for me. If the next GT does not enhance the offline player experience a lot, then I doubt I would get game or new console.
Paying full price again for essentially a GT Sport 2, but on a new console, holds very little appeal, and unless there was something huge added, I can't see it appealing to current GT Sport players happy playing online in cars and on tracks with great graphics. Time will tell though.
PS The thing about getting people to go to a next GT though is that you can turn off servers and essentially 'force' people to get with the new game, or move on, because there is not much left there without their servers.