That seems like a hard argument to make without any data.
Horizon ZD sold very well on PC, despite being originally a PS exclusive and being 3 years old when released on PC. Both Forzas seem to continue to sell plenty on PC. I think it's fair to say that there's at least a million potential sales for a Gran Turismo game on PC, and that doesn't seem trivial. The fact that Sony
continues to bring PS exclusives over to PC suggests that there's absolutely a market there for good games on PC (I mean, duh), but they want to squeeze every ounce of hardware sales out of the Playstation brand first.
Which is almost certainly what's really going on. If Sony/Polyphony wanted pure game sales, they'd be releasing on PC as well. The reality is that the console business is still based around moving hardware, and first party games exist to move hardware. With a bottleneck in the supply of PS5s, the only way to sell more hardware is to make sure that the warehouses of PS4s that they already had stocked up before the pandemic even hit are still attractive enough to sell at above bargain bin prices. That means that the PS4 still needs attractive new games.
If what you meant by
"making the game more accessible will ensure strong sales and that's why Sony have opted to do it" was "will ensure strong
hardware sales", then I agree. But that's not how it reads to me - you seemed like you were talking about optimising sales of the game itself, and that's not done by restricting the release to PS hardware only. GT doesn't "need" to be on anything at all, but if you're trying to sell the most games possible then you don't do that by cutting off what would probably be your third largest market. Possibly your second largest, depending how many PS5s they've actually managed to ship and how popular the game would be on PC.