I am not saying it would be easy, it would definitely be work that someone would have to do. I'm more saying that the Godot authors would not try to threaten you or consider you a threat, it's more likely they would want to help you. Of course as a business you would not do it if your customers weren't going to pay the cost to make it worth it.

I don't know enough about the implementation of C# and Swift to say for sure, but it does seem like the open sourcing of that would making it easier to do things like port some standard library component or algorithm over from one to the other, or perhaps do something like building a Swift implementation for the CLR.