Perfectly valid question. You are right, while there is a high chance that UE and Unity will be maintained, there is no guarantee. In any single person project, especially noncommercial, the chance it will not be maintained is much higher and Lumix Engine is not an exception. The only "proof" that it will be maintained is that I already worked on this 4+ years (as you can see on github) and that I'm making a commercial game with the engine.
If someone is not prevented by something (personal preference, license issue, features) there is no reason to use Lumix Engine rather than UE/Unity. UE/Unity should be the right option for 95% of developers.