Its true that its hard to sell an app directly on Google Play compared to the iTunes app store. A lot of devs have switched to in-app purchases or subscriptions as a way to allow their app to be free, allowing users to engage with the app before they decide if they want to buy in (the older style of doing this is a free app and then a premium unlocker app). Ultimately the pay upfront model hasn't really taken off and Android and probably never will for most apps.

You linked to a very insightful blog post.

I guess the TL:DR is that it's tough to get rich in the mobile app market unless you have enough users to make ads profitable.