> And if you hired me then as a mobile developer, I'd have quit, so you don't want to hire non mobile engineers into mobile roles.

That is a great point. I’ve also had the experience of working for a short time on a system, knowing that I would hate for it to become a regular part of my work. So hiring someone with experience is one way to mitigate staff turnover from undesirable tasks.

I don’t know Ruby. But I can tell you the difference between having the rails of the compiler and type safe language and having those rails taken away in a language like Javascript and Python caused a lot of heartache early on.

This is fair too but I think most experienced engineers for whom stack is a valid consideration have some experience with at least one dynamically typed language and one statically typed language and can self-select out of roles if they have a strong preference either way. And I don't know anyone for whom this was an actual blocker as opposed to an annoyance (in either direction) that they got used to after a while. On the whole, I think it's less likely for this to be a serious issue than, say, the details wrt how the system is architected and how the organization is run, some of which you won't really get to know until you start.