I noticed cryengine available on steam and started dreaming…
That would be an extremely serious undertaking. I’m pretty sure Unity is C#/JS, while CryEngine is C++/LUA. What this essentially means is that it’d be like translating the entirety of Atlas Shrugged to something like Japanese. Programming languages are just that, languages, and it’s very difficult and time consuming to port things over from one language and ensure that they work the way they should in a different language.
That’s the reason why no game is ever given a “simple” patch to stick it on an entirely brand new engine. There are a few minor exceptions, like Half-Life 2 being updated to function on Source 2007, but that’s because the fundamentals were virtually the same as with Source 2004. However, moving from one basic building block to an entirely new building block, and retaining a majority of the same features, is nigh impossible. Stuff is bound to break, and more time would be invested in bug fixing than actual development.
The developers picked Unity because of its licensing, simplicity, and flexibility when used correctly. I sincerely doubt that they’ll make the switch to something like CryEngine at this point in development. Choices on an engine are usually picked well after the conceptual phases, where the foundation for the general scope of the game has been decided. Rust is beyond that point, by far.