2025 or 2026 is still a bit early. 5 years is a reasonable expectation for current AAA development time. It has increased massively compared to older gens so how quickly GTs were released previosly is irrelevant. 2027 is probably the earliest you should expect the next mainline game, which would likely coincide with the PS6.
Edit: A few already mentioned these points while I was responding. But I'll leave the post up.
5 years is reasonable for an open world game built from the ground-up, ie Last of Us 3. But PD aren't building their title from the ground up, the biggest time sinks, cars & tracks, are carried over for multiple iterations. At most tracks would need a refresh with higher quality textures & vegetation. Much of the rest is additional content, game design elements, and technical improvements.
The closest open world example is Spiderman as the game world (New York) was iterated upon.
Spiderman - Sep '18
Spiderman Remastered - Nov '20
Miles Morales - Nov '20
Spiderman 2 - Oct '23
I'd be surprised if PD don't have GT8 ready 4 years after GT7's release. The only reason to delay further is if there's something akin to a GT7 Remastered that takes full advantage of PS5/PS5 Pro. PS6 is not expected until late '28, so we're a good 4+ years away. PD will need a new product on the market well before PS6 releases.
We are gonna get another 5-10 proper Gr1/Gr2/Gr3/Gr4 race cars at most till the end of the GT7 lifespan, while the remaining few dozens will probably be mostly road cars and maybe a couple of racing cars belonging to other categories.
There's no way to judge what PD have planned for DLC. We could get a Super GT GT500 pack with the latest Toyota, Nissan, Honda. Race cars are more suited to themed packs, they did something along those lines in GTS, plus the latest Super Formula in GT7.
Other than a few rally cars and the odd race car, the datamined lists doing the rounds don't tell us much about contemporary race cars (or road cars for that matter). Unless I'm mistaken they don't even include classics like the 88C-V, 905, GT-ONE, R390. I've no doubt PD are working on these alongside the latest models from Hypercar, Super GT, GT3 etc. But these will be highly guarded, the moment any data leaks it would be all over social media.