My hypothesis is that not all tasks in a software project require the same level of skill or domain expertise. Context: I'm the technical cofounder and (currently) sole developer of a tiny company developing SaaS applications. The core of these applications definitely does require my domain expertise. But other things, like the purchase UI and account administration, just require one or more programmers with adequate skill and a strong, reliable work ethic to deliver (and maintain!) good enough implementations. Perhaps this suggests a two-language approach for such applications, with the domain-specific core being written in a more niche language. But polyglot projects bring their own complexity that makes it harder to eventually hand off maintenance.