Thanks vornay for mentioning Okijin Games
Honestly, my experience tells a different story. I have been making a living exclusively from this business for the past 3 years and I am confident that there's many more opportunities ahead.
First, the thinking that you will necessarily make more money on bigger marketplace than smaller one is wrong. In the same vein as elevator's speeches where the business owner tries to convince investors that because the market has 1 billion potential customers, if he gets only 1% of it they would be rolling on the money. In reality, larger marketplace means more competition, higher production values, higher budgets then much harder exposure. I could run a campaign to 40K installs on Windows Phone for $500 in 2013... How many new players would I get for that budget on iTunes or Google Play? The size of the marketplace only matters for the bigger companies. For the rest of us, we should go where user acquisition is the cheapest, that is all that matters. And if we ever reach that point where we need a bigger marketplace then I would say this is a "nice" problem to have for any business!
And then this is just the surface, you need a game that can pay you back. What's the expected ARPU (average revenue per user) for a game made in 2 weeks and a "Remove Ads" Button as entire IAP strategy? The reality is that your only chance to cash in is to pray the gods for your game to go viral (so you don't have to spend a dime in user acquisition) - and this is a strategy that could have you waiting for a life time, very similarly to people playing the lottery (and I am not saying don't try). ARPU for a game like flappy birds is at best 0.01 (and yes with 5M daily users it comfortably pays the bill) but if the developer had to pay for user acquisition at that scale he would have probably lost money at the end with that game.
True, HTML5 games quality is a step behind but is good enough to compete in several game categories (puzzle, casual...). I am confident that many players cannot really tell the difference between native or not with my latest game Sailor Pop, just released on Google Play. We are catching up
Finally, if you have any game idea I would encourage you to just go for it and make it a reality and release it somewhere - anywhere. Don't waste too much time in existential thoughts about which technology and best place to release, go with what can make your project a reality. If you meet success, then again, all problems will be nice to have (porting to another platform, growing user base etc). People have been making much money with only targeting one platform. In Korea where I live, there are game businesses (SundayToz...) making millions mainly targeting the local KakaoTalk platform (the number 1 messenger app there).