That point here that made to me makes this a pretty open and shut case because you can still protect your IP interests. The only “open source” part here is whatever surface area the and engine have. Epic would have to be better at deprecating public facing APIs (instead of removing it outright which has happened) to ensure that the source still worked between releases. I wrote the RadiantUI framework and I thought my support for handling arbitrary polygonal meshes for UI interaction was pretty neat. During the time when I was selling it if I’d really wanted to I could have isolated that part of the source in a separate library.
I encourage Epic to adopt this since it really does alleviate a significant issue and over time it will become a major thorn that could stunt the viability of marketplace plugins. As an Epic customer if my project breaks during every upgrade or I have to wait for weeks for a half dozen authors to catch up it can get frustrating and lead me to avoid using them in my project. I don’t think Unity’s marketplace would be as vibrant if they had this kind of issue. It’s very nice to be able to move through Unity versions fairly smoothly in-terms of third party compatibility.