How much time would it take for a smart, competent and self motivated person to get up to speed on this new but related field? Given that number of hours, is it worth accepting that period of relative 'unproductivity' in order to hire them?
That seems like the right heuristic to answer your question.
Hiring someone willing to learn a new field filters for generalists and people who need jobs. Neither of those qualities are bad.
Specialists, or people in love with a stack, don't want to use 'some other tech'. As a senior, I pick the stack, because I have an idea where the industry's at and what's best for the type of software I'm experienced with making.
These hidden filters are interesting.
This depends on the position; smarts are not a replacement for (domain) experience, because certain knowledge can be learned only through trial and error and/or through long-term exposure to a domain (in other words, experience).
I believe that domain experience is the more necessary the higher level the position is (e.g. seniors who can decide on architecture, leads, architects...).
Having said that, I certainly agree that in the (hiring) industry it's common to be excessively attached to the specific tools used; I also suspect that smaller companies tend to think like this more than bigger ones.
I think the more appropriate question is about the relative effectiveness of someone who has been leading Android development teams for 10 years and someone with the same experience but in a different (but related) field.
GUI libraries are pretty similar to each other. I have never written an Android app before, but in a hypothetical case when I would want to make one, I’m pretty confident it would take me a week or two to learn these things. I’m very familiar with quite a few other platforms: everything Windows from WinAPI to UWP including phones, iOS, I made my own GUI frameworks even…
If that lead developer does not have _any_ experience writing rich GUI apps in any language, and no experience with computer graphics either, now that would be a red flag for me. That’s too much new stuff to learn in reasonable time: computer graphics in general with these pixels and vectors, Unicode shenanigans, UX shenanigans especially touch screen related, and quite a few others.
The thought might be, how long would it take you to make a great one? An 'expert' might be able to create it by the time someone else has started figuring out lower to mid-level concepts.
Mid.level concepts are more diverse, but not dramatically so. All popular GUI libraries are object oriented, web included. MVC on iOS does have some differences compared to MVVM on Windows, but conceptually they’re still very similar.
What’s that? If you gonna mention Java shenanigans like memory management and garbage collector, .NET is fairly close. Mobile-specific things like multi-touch UX and power management are shared by all battery-powered devices. Many kernel-specific things are shared with all Linuxes including desktop, server, and embedded.
> you probably wouldn’t do a good job building the foundation of a large app
I probably will. I already did a few times in my career. When I found that iOS development job a decade ago, I had zero prior experience with the platform. I had many years of prior experience developing stuff for Windows, game consoles, and some older mobile platforms.
I assure you a million companies did exactly this ca. 2010. And that was back when it was much more difficult to write one, both devices and the SDK had orders of magnitude more sharp edges and restrictions than they do now.
A senior without android experience is going to have to learn the platform; this will take longer than learning the language. The pace of change updates the best practices often enough to serve as a kind of catch-up mechanic but native mobile does in fact require domain knowledge. I would be comfortable putting a senior iOS dev on native android but certainly not as the lead.