What the market wants ("Swift developers with ten years' experience"), what we as labor want ("Fun projects and a paycheck for whatever we make"), and what's actually required ("Wordpress ecommerce theme that's mobile friendly and doesn't look like complete ass") are all different.

The specific demand is invariably for a particular technology or platform, because management wants to solve their pain point right now and want to manage risk. The problem is that those experts are not super useful in five years when the platform changes (or three years, or six months).

Better to be an expert as using different technologies and getting up to speed quickly.