You don't mention that while you are still selling the current game. And you do not mention the length of support for that game either, especially when it is connected to the support for its content. Buy GT7, because our new game will be out in 2 years time!
For me, the slow down of additional content each month, even compared to GT Sport, is an indication that they are working on the next game. Whether that will be on the PS5 or Sony's next console is anyone's guess. I personally think the next game only being on the PS5 would be a very hard sell. Without an increase in the technology of what a new console brings, what would they next GT offer over GT7 with the same technology running it. Seemingly gone are the days of significant software improvements during the lifetime of a console. Is the PS5 offering a massive difference to most GT players than GT7 on PS4!