Backports work because the 13.4 firmware (or whatever wolverine needs) has been exploited. The encryption keys that are part of 13.4 have been dumped and extracted using that exploit, so now you can tell your 4.0 ps5 to tell Wolverine to "decrypt the game with this key" and it'll do it. If GTA needs 14.0 keys, you won't be able to do that until someone cracks 14.0 and dumps the key. You could give GTA the v13.4 keys just like Wolverine, but GTA doesn't decrypt with 13.4 keys.

Similarly, pirates can create decrypted dumps of games (I don't know about ps5 but definitely on other consoles) by passing the key through during the dump process. That still requires having the key though, and getting the key means exploiting the firmware that distributes that key.

People will not be able to play it without updating their PS5 firmware (which for most people happens automatically). You can't downgrade firmware versions. So by locking the game behind the latest update (a version that currently blocks piracy and homebrew), the game can't be pirated via PS5s until someone finds an exploit on a version that can run GTA.

This is how every game on PSN or discs for PS5 works. Disc copies are usually a month or so behind 'latest' due to pressing time. The last big ps5 release keyed to a vulnerable firmware is Marvel's Wolverine.

Piracy on PS5 is always a game of catch-up. Exploit a ps5 firmware and you can pirate anything keyed to that firmware or older, but you can't pirate (or buy) anything newer without updating the console and losing your exploit. Eventually, newer firms get exploited and the "pirate range" gets expanded. But new exploits take a lot of time and effort to find and it gets harder every time a vuln gets "burned" - I don't think the GTA firmware will be solid forever but it'll almost certainly last long enough that the game's PC version will be out.