Programming language theory does not study "advantages;" it's simply not a question it asks, let alone answers. The theory is concerned with what properties certain formal systems have. It cannot, nor does it attempt, to assign those properties a value, just as mathematics does not ask or answer whether prime numbers are better than composite numbers.
> None of these things is straightforward to control for, and I'd be extremely skeptical of any quantifiable metric someone shows me that purports to show clear wins in real-world software development based on language paradigm of all things.
Maybe, but that doesn't matter. You cannot claim that you're providing a significant benefit and in the same breath say that it isn't measurable. An advantage is either big or not measurable; it can't be both. If you say that the benefit of the language is offset by the bad tooling, then you're not really providing a big benefit. If and when the tooling catches up, then it's time to evaluate.
But maybe not. I find it very dubious that significant differences are not measurable in an environment with such strong selective pressures for two reasons: 1. it doesn't make sense from a theoretical perspective -- adaptive traits should be detected in a selective environment, and 2. it doesn't fit with observed reality. We observe that technologies that truly provide an adaptive benefit are adopted at a pace commensurate with their relative adaptability; often practically overnight. The simplest explanation from both theory and practice to why a technology does not show a high adoption rate is that its adaptive benefit is small at best.