As I posted on Mastodon:

Not enough. Even this sets precedent that #Unity or any other middleware provider can retroactively shake down its past customers for more money. One penny would be too much.

Any change to an agreement like this needs to be contingent on the customer doing something like updating to a new version of the software or renewing their subscription, and the law needs to make this explicit (if it isn’t already; IANAL but I’m not a lawyer.)