> The experts in how to profile/optimize/etc a system aren't going to be JS devs though. They're going to be people who are used to dealing with systems that need to be written in languages from the machine-code-compiled lineage.

Sure they are. The skills aren't really language-dependent, and nowadays machine code is so far away from the actual hardware behaviour that it doesn't actually help a lot. Besides, the biggest speedups still come from finding errors or inappropriate algorithms or datastructures, and that's if anything easier to spot in a higher-level language where there's less ceremony to get in your way.