How is this any different from the sunk cost that's a consequence of going with _any_ particular technology stack?

I mean, if you chose Python/Postgres/etc., presumably, you'd have to rewrite/retool everything if you went to a different stack.

If you're talking about open vs closed source: the C# compiler (Roslyn) is completely free and open source, as are a number of .NET components: CoreFX - Core foundational libraries, CoreCLR - runtime.

As well as PowerShell, VS Code, Typescript, ASP.NET Core, MVC, Blazor, F#, Z3 Theorem Prover, etc.

And that's from just the first few pages of their Open Source page: https://opensource.microsoft.com/