I don't follow this logic. Why not use C all the time by default then?
But why not get all the gain? Well, sure, if performance is the only thing you care about. But it usually isn't. You might also care about networking, or multithreading. Would I rather write that code in C, or in Go? Is it enough better in Go to be worth that last 10% of performance? Arguably, yes.
When someone says "X matters", it doesn't mean that they are saying "X is the only thing that matters". It is not correct to conclude that they should, for consistency, be saying "turn the X knob as far as you can". Instead, interpret it as saying "given your other constraints, look for a sweet spot where X is somewhat toward this end of its range."
That's why saying a compiled performant language should be default is not logic I understand. Default should be whatever you need for your requirements. If you don't need anything special, just go with what you already know well.
And, as I said, "Why not use C all the time by default then?" does not actually follow as the logical endpoint of the argument that your trying to answer, so making that your response really doesn't contribute much to the advancement of the conversation.
But if you're writing server code, I think choosing an interpreted language like Python/Ruby is the single worst mistake you can do.