One of the things that developers need to focus on when developing their apps is how are you going to monetize it? Most developers think "ads, ads, and ads" but you need to be looking at monetization models, ads are a monetization model but it's not the only one out there. Ads are probably the easiest way to earn revenue. But there are many other ways that don't put banner ads in your beautiful new app, and still drive in some cash, so you can still put food on the table.
The biggest app monetization model is freemium. Basically what that is, is a free app but with in-app purchases. Most games are now freemium with in-app purchases. Whether it be for different items, or more energy to play longer, or anything else. That still drives in cash, and you only have to give a certain percentage to Google Play or whatever app store you're using. When doing ads, platforms like to take out bigger percentages from your revenue. Which can sometimes mean less money, over the course of your apps' lifetime.
Other popular models include affiliate marketing. Obviously that's a bit tougher in a mobile game, but in an app, you can easily do affiliate marketing with strategically placed banners that are small and not overly large, inside the app. Usually how that works is you get paid for having it in your app for a certain length of time. Which means that it won't matter how many clicks the banner gets, or how many products are sold. While these still look like ads for many people, they are actually much better for the user experience. As you aren't using all kinds of flash, and other resources, making the app take longer to boot up and be used. And often times a lot of users don't even notice these.
So if you are a developer in the planning stages of a brand new app. Make sure you think about monetization models and not just ads. Also make sure you do your homework. While ads are a very popular monetization model, it may not be the best for you or your users. Which is why it's important to look at all app monetization models available out there.