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.