I’m pretty sure devs can just withold payment after Jan 1, and for games already released if Unity wants the money they would be forced to sue the dev for not adhering to their illegal and unenforceable contract. They would have to prove the validity of their per-unit charges without having actually ever measured the units.
Yeah, so in this hypothetical eventuality they rob themselves of income in relation to games already released, as well as goodwill in relation to games in production or planned for production in Unity. Seems to me like a recipe for backing down on one or the other.