But that only works if Unity open sources the engine. A ubiquitous engine is not the same as an open engine. The fact that we now have jDoom, Doomsday, and other enhancements to the engine that let us play Doom, Doom2, Hexen, Heretic, etc. only work because id software released the source code. Otherwise, it would be like any other proprietary engine, and we'd have to reverse-engineer it if we want to be able to play those games again.

I'm sure you could argue that Unity is currently more cross-platform than engines were 10-20 years ago and already works on Mac/Win/Linux/Android/whatever, but we can't guarantee it will be around in 20 years.