To say that you can NEVER make anything if an alternative version already exists is just not realistic. The SketchUp Extension community has grown and thrived for years by ideas going between developers, getting fine tuned and offered in new/different ways.
I do not see SketchPlus at theft. I DO see refusing to run if someone else’s extension is installed as a poor choice. SketchUp is a collaborative, open platform. It is part of the reason that people use SketchUp.