Developers can invest a significant amount of time and resources building tools, supporting systems and workflows around these engines that aren't necessarily portable to other engines. There is also a significant time investment in learning the API, engine internals, and nuances of operation. Disrupting revenue streams so aggressively for the licensees is only going to result in breaking trust between Unity and their customers, driving them away permanently. They can also forget about taking on any new customers as few will be willing to move to a platform which tries to aggressively limit their choice of income model.
While Unity has filled a niche in it's usefulness, there are much better offerings (either higher or lower on the spectrum of capabilities) for other proprietary engines and a few under-featured free engines. They are not irreplaceable.
I do feel bad though for all the Unity developers that are having to deal with this but at the same time I think that many might have anticipated something like this would happen for some time.