I'm not implying that C is objectively inferior. Quite the contrary: there are tools that suite a specific problem. Maybe Twitch did spend money tuning Go, maybe that was a waste. Point is: they go a free performance boost thanks to a language the prioritizes what you want to do instead of how you do it.
I'm a big believer in early performance optimization, premature if you will. Though, ultimately, what I can crack out in a day with C# is worth months of C. Iterate, even with languages. If, after profiling, you find that your expressive (as opposed to explicit) language is wanting it's time to iterate into a different language. Get something competent out of the door, spend more money on it only if you have to. Right now our socket library consumes less than 1% of the CPU (GC and all), until everything else catches up there is no benefit to getting any closer to the metal.
I'm afraid the truth of these things is that if you try to squeeze maximum performance out of some of these more sophisticated languages, you have to be able to understand and debug the runtimes that come with them. Not that many people are up to that, which means the hurdle can be higher.