The funny thing is that when I first brought home a book on Cocoa programming in the mid-2000s, my dad (also a programmer) saw it and gave me almost an identical line to your last sentence. He said it was a waste of time, that I'd never make money doing it, and the market for Mac developers was tiny.

I've been doing Mac (and sometimes iOS) development almost ever since and as of right now make almost 2x my dad's career-high salary. That tiny market has treated me very well.