The Microsoft stack by and large has a very closed ecosystem. You can use it in a free environment, but that is very much not how people actually use it. The .NET ecosystem is huge, but it hardly ever touches the freer ecosystem most languages exist in nowadays. If you use .NET "on a fully free/OSS stack," you're more or less in the wilderness, because the other Microsoft stack people aren't doing what you're doing and neither is everyone not using the Microsoft stack. It's basically a worst-of-both-worlds proposition.

However, I don't believe anybody actually thinks "anyone who uses .NET is ultimately inferior." Anyone who would express their distaste for the Microsoft stack that way is obviously trolling.