They want money for the work they did to improve the game for a different console than you bought the game for.
The wider market will decide whether this model is going to fly going forward. Like Oblivion's horse armour. There should be a discussion predicated on the commercial realities. And by discussion I do not mean fanboys, I mean people will vote with their wallets and Sony will know who bought it on PS4, who has since upgraded to PS5, how many people are willing to pay to upgrade and how many all of these person play the PS4 and PS5 versions and how often, and whether this move was resulted in more play time.
It's a lot of numbers which we'll never see.