The remote development plugin in particular is potentially very commericially valueable.

GitHub Codespaces is a paid service and they don't want any competition. Charging for Visual Studio every two years is old hat. Charging companies for their IDE by the developer-hour, now that's Thinking With Cloud.

Partly. They've also said that they used some code from Visual Studio in some of it so I guess getting corporate permission to open it is not trivial.

For the Python one at least it's not as bad as it sounds - the Pylance extension is closed source but the core type checker Pyright is open source (and much much better than MyPy).

See also https://isdotnetopen.com