It's better to be good at mobile than to be good at nothing at all, sure. But I'd be really careful about picking a specialty that amounts to "my domain of expertise is AppKit", or "my domain of expertise is Scala", or anything like that.
You could claim that software security is "just programming" for instance... But you likely wouldn't, because it requires a level of understanding that is especially deep, and niche. It doesn't really matter that a security expert likely reads a ton of code, and writes some too, it's that they understand a subset of the industry to a high enough degree that they stand out among their peers.
I propose that you can be a mobile expert, just as you could be a VR expert for example. At the end of the day it's more then just programming, it's a combination of hardware / software / tools that retains a degree of uniqueness.
Yes: you can specialize in being a really good programmer for a particular platform. I have a lot of friends who specialize in being Serious Engineers, who understand how to get interesting and important things done with lenses and how to effectively get a whole project away from null pointers and into monadic error checking and how to use type systems to prove programs mostly correct as they're being written.
That is a real specialty. It takes real skill and investment of time to get to the point of being expert with that stuff. I'm not diminishing the value of being an expert software developer.
I'm just saying that basket of specialties is unlikely to command a very high wage premium, not because it's unspecialized, but because it's hard to fashion a value proposition for it relative to your competition in the market.
There will be times during which particular platforms or languages will get hot, and it'll be good to be expert with them. But (a) those phenomenon will be more transient than other problem domains, and (b) the wage premium for being specialized in them will still probably be lower than the wage premiums for the other domains.
At any rate: as long as you're thinking about the domains you're specializing in and not a career as a "programmer" or a "builder of software", we can probably agree to disagree about the rest of the details.
To make matters worse, developers will disagree with each other about what the Right Answers are for a given platform or language or engineering approach.
Cryptographers (as a counterexample) also disagree with each other about the Right Answers in crypto. But serious crypto people are generally competing for jobs only against other serious crypto people, and there aren't that many of them.
Moreover, the general domain of cryptography isn't going anywhere. But look at Objective C, or Windows Mobile.
Finally, proving value as a subject matter expert on a language or platform depends on companies having hiring processes that reliably spot expertise. But, of course, hiring processes do a terrible job at that.
I agree with this entirely. But it makes me realize that perhaps I haven't explained my position well.
I would agree that being a mobile expert is likely going to be LESS lucrative than being, say, an expert in recombinant DNA technology with a fluent C++ background and a PHD in machine learning.
The only issue I see with advising people to invest tons of time and money into niche fields, is exactly what others have pointed out in this thread - You can't really predict what's going to be valuable in 10 years.
So take your pick - Go the safe route and become an expert in a more general field like, for example, mobile. Or pick something incredibly niche, which will shrink your competitor pool and increase your relative value proposition. A classic risk/reward scenario if I've ever seen one.
I don't think this is all that safe, really. What we consider "mobile" development has really only been around for about a decade with the releases of iOS and Android. (Sure, there were mobile platforms before that, but most people didn't really care about them, and specializing in them would likely limit your options, not expand them.) In that time, mobile development has gotten nothing if not easier. So, over time, being a "mobile dev" will become less and less of an important specialization. More people will do it, and the frameworks around it (both those released by Apple/Google, and third-party things on top of it), will make it easier and easier. By that point you might be an "elite" mobile developer who can do certain things on mobile that most others cannot, but the market for that level of specialization will likely also be small.