I also asked the question, and got an answer. The hypothesis is that they'll release new versions under a different license, also meaning that if the devs never agree to the new license, they'd avoid the fee. Of course, that would mean that any engine level bugs in their game would become unfixable. This also means that large developers would be exempt, as they likely have contracts in place that supersede the license agreement.

Im trying to think like a money hungry, out of touch POS CEO here.

Unity uses a subscription model right? Where each year you have to renew it and agree to new ToS. Well if they just put in their new ToS that companies have to pay retroactive fees and that company "agrees" to those ToS, then that means it's not illegal since they technically "agreed" to it...

Hope to he'll it doesn't hold up in court but if Unity goes through with this who knows.