Off the top of my head, say you have a PS5 and your kids only have a PS4…Lol what? If you have the PS5 version, why would you ever want to play on the PS4 to begin with? That doesn't make any sense.
With the PS4 version on the other hand I totally get that, because it's nice for consumers to know that once they're able to get a PS5, that they can upgrade to the PS5 for free/$10 (Well the $10 upgrade fee being normalized still sucks, but yea)