Well, it depends on how you think of the license. Is it like you buy a license for a specific release or do you subscribe to a license to the software across all releases?
I don’t know. I can see how it would be ridiculous if Amazon said “oh, by the way, starting next year you have to pay a cent every time you finish any of the books you bought on your kindle”
But if Netflix went “starting next year, there’s a surcharge of 1 cent per episode you watch” nobody would go “surely it can only count for episodes released from next year!
Which raises an interesting question to me: what if a developer wants out of the Unity contract? Does that mean they have to somehow break games consumers already purchased so as not to be liable to install fees?