No, it's a dick move to expect someone to buy another console to access extra content for a game on a console they already own. Just like it was a dick move to require people to use a mobile device to access the track creator in GT6. We're not all made of money. If I buy a game for PS4, I expect all of it to work on PS4. Not just bits of it.
If they want to force people onto PS5, make it a PS5 only game. Don't catfish people into buying a PS4 game and then dangle extra content that's only available on PS5. That's deceptive. It hasn't stopped other companies doing it, but I'd like to hope that Polyphony has a little more respect for their players.
You notice how your definition of "everyone" here doesn't include the actual players? PS4 players do not win from this scenario.
Hide your love of giving your corporate overlords money better.
They don't, because no Gran Turismo game has received updates anywhere near as long as GTA 5. Gran Turismo 6 is roughly the same age as GTA 5, and Polyphony shut down the servers on that one
nearly four years ago. If GT7 is still going as the latest iteration of the franchise in nine years, then sure, they can reasonably look at discontinuing support. But we all know that's never going to be the case.
If there's a Spec 2 for GT7 that introduces new features, it's expected that they will offer the same for the PS4. It's not expected to be performance equivalent, but it is expected to be feature equivalent. By choosing to release on two platforms they've locked themselves into supporting both of them. If they want to move to supporting PS5 only, I'd suggest that they release GT8 as a PS5 exclusive.