Embarrassing way to run a gaming company. Free update, make it free
The problem is how current PS5 upgrades work on the console. They're not really upgrades, it just gives you a free PS5 version of the game. So if they were to offer the PS5 "upgrade" free to anyone who has the PS+ version of the game it'd be granting them a free PS5 copy that's not tied to their PS+ subscription, meaning they keep it forever.
The solution would the make PS+ PS5 upgrades that are tied to your PS+ subscription, but they'd probably just rather make people buy the PS5 version instead.