Honestly; I've made money on most platforms. I've launched Android, Windows, iOS and Mac apps. As long as people are using the platform, it doesn't really matter. Smaller platforms (such as windows phone) have the advantage of less competition. I know a guy who was making $40k a month from a Palm Twitter app a few years ago.
Example: https://itunes.apple.com/us/app/ventrilode/id486115720. That's an app I launched 4 years ago.
It brought in hundreds of thousands and took me 2 months to develop + the occasional bugfixes and small updates. Probably 3 months of work over a 4 year period.
> Do they keep generating any sort of revenue after 6 months?
Depends on the app. In most cases, it's not worth investing an additional 2 months developing/improving an app if you can build it 1 month. Launch an MVP and see if it sells. You can always improve it then. But once you have an MVP ready, it usually doesn't take much more time to "finish" the feature set of an app. That's what great about apps; as a project they can be "finished". Some apps sell for years, others only last 6-12 months. Also, for some apps the money comes in fast, for others it's slower but they will last longer.
You just have need to find a target user group of significant size. The app I linked above integrates with a system that, a few years ago and at the time, had around 10m daily active users. It's a niche communication system.
That means a subset of those users will automatically search for your app if you get it to pop up for the expected search keywords ("app store seo"). You're tapping into that user group without having to spend a dime on marketing.
Find a user group with lots of users - people will buy your app.