"leading several video-game makers to say they would boycott Unity until the policy is changed" - if anyone built an implied conditional return to Unity into the language of their letters of condemnation and goodbyes, I think those people were a minority. It seems like I heard almost everyone say that Unity can't and shouldn't ever be trusted again.
I thought that that sentiment, the thought that Unity will stab you in the back at any time, was part of the reason most people were making the extremely difficult switch to another platform.
If the most important thing to them was escaping the installation fee by any means necessary, they would have led with demands. Because they knew Unity suddenly meeting those demands would mean that they could cancel the expensive engine switch. Everyone who offered Unity an ultimatum did so because they needed to, everyone who didn't offer an ultimatum was able to switch platforms, and still is, and still has so much reason to, I think expecting them to take the short-term cost savings of not switching is unreasonable.
Unity is a sinking ship but most game studios are not. They can think about the future, about trust, and switch away from Unity now.
I expect Unity to continue to lose users, just a little slower than yesterday.