I got a firsthand demo of this yesterday when I wanted to see how a .NET DLL full of CIL bytecode was put together and how it fit into a larger application. I asked Opus 4.6 for a disassembly, and it basically regenerated a fully-commented C# source file that looked like it came from the company's private repo. Amazing.
Too soon to say "Everything is open-source now," but that's where we're headed.