C# language is less than 10% of everything that comes with embedding C#.
So when you say “native c# support” I feel very lost on what you really meant. If you mean just the language, Epic could refactor Unreal Script to read like C#, no need to add NetFramework to Unreal then.
NetCore or Mono, the stuff that runs C# dlls, they are HUGE libraries. And everybody will make cheat mods for your game btw, because it’s really easy to crack and patch C# DLLs.