I personally can't think of an all-rounder language that is better than C#

I didn't ask about whether it was good.

I asked about whether it's past its peak.

It's certainly reaching "maturity". And I'd say it's improving still but the improvements are small (which is expected at that point).

I wouldn't say it's past its peak because it's still improving and there is no good alternative for a language of its class. Go isn't it (I doubt there will be a good desktop/mobile app/game engine etc story for Go in the future). Swift could have been a competitor in the allround space but Apple doesn't seem interested in conquering the world outside its own garden. I'm not sure who it would be that would make the "next" C# and .NET. Only Microsoft and Apple are making commercial desktop environments, for example.