It's not about whether one can pick up the newest fad, though. It's about whether you can convince an employer of that.
In a lot of cases, it doesn't matter if you worked through the tutorial, have used the droid simulator, and can talk about the differences and most important API calls. Manager: "Sorry, it's not Java. You need 3 years professional experience in Kotlin."
We've all been there and maybe it's because there are so many "coders" who only know how to do one thing and where each new NoSQL db takes weeks to learn. Or it's because there are too many b-school hiring managers.
I have a slightly different take on this. I don't dispute the facts - just the interpretation or the emphasis.
You don't staff a 10-people team with 9 tech-SME's, or even 5 tech-SME's. At most, a 10 people team will likely have 1 tech SME, or maybe 2 at max. Finding those SME's is often very difficult, especially in new(er) technologies. If you don't find someone who is truly an SME in that very niche technology - then you are better off using one of your existing in-house tech-superstars have him grind through the learning curve. Hence the take-no-hostage approach to the recruitment of these positions. You are looking *only* for uniquely qualified people in these positions, don't care for people who are just "qualified", and you cast a very wide net for these positions because they are so difficult to find.
These positions create a huge amount of splash. But irrespective of the amount of noise they create, they are only a tiny number of total jobs out there. A vast majority of positions are not filled this way.
I have changed jobs externally (i.e. moved employers) 3 times, and moved internally many more times than that (between 2008-2010, when I was hanging on for dear life, I moved internally 6 times in a 2 year period. It seemed like a never ending cycle of find a position, oops thats gone, now find another one!!). My moves have crossed roles left and right (think developer, project manager, management consultant, quant). ALL of them have been done through networking. Not a single one of them was because I was the expert on a specific technology. Heck, in the current job I have, I had never coded in the language I use right now outside of as a hobby to try it out. I was not even in a "technical role" in my old job before I joined this. I was basically a "management consultant". I had a significant amount of domain knowledge, and was hired based on that into a technical role with the expectation that I will take a couple of weeks to pick up this new language while overseeing a small team who all were better coders than I was in this specific language.
Now, my experience is also not an "average case". But the point I make is that vast majority of hiring happen between the two extremes of "tech skills don't matter" to "need uniquely qualified SME". So I am not quite convinced it is as big a problem as it appears in the first glance if you only look at those high-noise jobs on all bulletin boards that are looking for uniquely qualified SME's.