I think they might wait until they know the GT7 servers are running well enough and can support the number of players that will be on them from the start, first; then they'll shut them down.
I've watched several videos where they said they fully expect them to shut down the servers the minute GT7 goes on sale. Which is their prerogative. But I think it'd be better to wait until they know the GT7 servers are fully functioning before doing that. Also, not every GT player will have GT7 AT launch. I know I probably won't. I'm waiting for my income tax refund to show up in my bank account. That way, I can buy a PS5 and GT7 at the same time (plus 1 other game I've been waiting on).