I disagree. I think companies ought to develop for every platform, companies ought to create a market, but developers ought to specialize generally.

I don't think most developers are capable of being excellent at iOS development, web development, Android development, desktop development, etc.

Certainly it's not advantageous to specialize only in a particular language (particularly a dying or dead one), but that's quite different from a specializing in a particular platform.

If they have worked with developing in these many areas, they'd likely have some experience in each, but be not experts in any -- there are just too many.