nowadays you can probably rip most of your game from the Unity toolkit and marketplace

If you were developing a Unity Engine game on Monday, you did so with the general understanding that you wouldn't be charged additional royalties or fees beyond your subscription to the Unity Editor software itself. If you were developing that same game on Tuesday, you were suddenly subject to shocking new terms that would impose charges of up to $0.20 per install (starting next year) after certain per-game revenue and install thresholds were reached.

This change led to a firestorm of understandable anger and recrimination across the game development community. But it has also led some to wonder how such a massive change is even legally possible. Can Unity just unilaterally alter the fee structure its developers were relying on, even for development projects that were started (or even completed) under completely different legal terms?