I didn't downvote, but IMO it's only half true: C# and the runtime really are excellent. But the C# library ecosystem is much smaller than the Java, JavaScript, PHP, and Python ecosystems (and even Go/Rust for a lot of things), and there are even quite a few commercial (paid for) libraries! This may not matter for a particular project, but for me it's significant enough to not make C# a universal recommendation.

Hopefully it changes now that C# is open source, but for now it's quite a big downside.