there's no logical reason to move.
why would anyone want to switch to a worse codebase with fewer assets and more obstacles to overcome, especially when the game engine is being made to mimic source in every way?
you would be moving to a worse product for the sake of being on the "latest" platform. GMod, though as awful as it can be, works for the most part and has a working content distribution platform.
so unless S&box adds some completely revolutionary new feature that isn't just a copy of something UE4 comes with by default, and isn't some useless gimmick, then I don't see any advantages of migrating, and therefore I believe the answer is almost certainly "no."