Nearly everything that is open-source depends on people doing work for free that is used by other people who use it for profit. Maybe OP is making profit through the software they're writing using C#. Should Microsoft charge them for C# usage? Microsoft is the ones paying for development costs after all.

The purpose of open-source software is to be able to fix problems for your use case and continue using the improved software. OP could have just chosen not to upstream their changes (I'm assuming you'd be okay with that since Microsoft doesn't get their hand on the fix) but then they wouldn't be able to easily update their compiler.

I guess the only real option here is to use a community-developed language with no for-profit company behind it. Then write all the libraries that Microsoft wrote for C# and published for _free_. Hmm.

I wonder what percentage of popular languages / libraries out there have the level of reliability & support that C# and other open-source software with big corps behind them has.

But this way you are promoting inequality. Being able to commit substantial amount of time to volunteer without remuneration is a domain of privileged people, usually from wealthy white background. They can they use their contributions as highlights in their CVs and for instance get better jobs than people who were not in a position to give their time for free. Hencewhy in many countries, like mine, things like internships have to be paid. If they weren't then positions were mostly filled with youth from wealthy families.

If Microsoft was honest about equality and open source, they would have donated the whole platform to a non-profit foundation and removed its name from it.