That would requore basically remaking the game from scratch, i believe. And thats assuming the game is even *possible* to make, in a different game engine. I'm no expert, but I know making games is very complicated, and certain mechanics and features can often only be done using certain game engines.
So even if the developers *were willing* to switch game engines, and spend months and months trying to recreate their game, in a totally different method and programming language, there's no guarantee it would even be *possible*, unfortunately.
For the time being it seems they can survive and still use unity. So we will just have to hope things work out.