Stock photography is still a viable way to earn some passive income but to get good in it is just the same as any genre of photography; it takes years and years of hard work to learn the market, the system, built the portfolio, and eventually you start making money but might find actually you don't enjoy making that money afterall coz you ended up shooting for the market than for your own creativity expressions.
Refer to Catchlights' post again. That shot of morning mists in Singapore is way rarer, harder and way more beautiful than the Chinatown snapshot, but Chinatown shot sells. So go figure.