My first line of thought is that developers push out a game, rake in piles of cash and figure "that's it, I'm good". Okay, that's pretty understandable. I mean, they probably spent the last half a year working on something and it probably got pretty hectic towards the end. They want a break. The problem is what happens after that. They lose interest, work on something else, probably push out the odd bug fix, and eventually abandon the project.
However, what they could have done was have a content update ready for deployment, even before the game is released. If they would plan ahead a little, they could serve this update at the right timing to keep people who are finishing the release-time content interested. After that, have a roadmap. Plan to always be a month or less away from the next content update.
Why would you want to do this when you can just roll in your newly acquired pile of cash and move onto a new project? I can think of a few reasons:
If a gamer buys your game and before they're even done with it, there's a new content update that keeps them going, they will remember that. They'll tell their friends about it, who will buy the game. You'll build a following not just for that game, but for your company's name. In this current climate, you'll definitely stand out. The next game you put out will probably sell more on the initial week. You'll likely get written about online (by Touch Arcade, etc.)
What better way to make someone's mind up about a purchase than to know that they'll be getting more and more content down the line. And when you deliver as advertised, not 3 months later, if at all, your promises will be trusted. Often I've seen developers promise content updates that either never happened or happened so late that nobody even noticed or cared. Why work on a content update that you aren't going to be paid for? You just wasted your time.
Provide some free content and sprinkle in some IAP stuff. If people are getting all the updates and actually playing them, they will most certainly want to buy the little extras, be they maps, levels, weapons, new classes, etc.
As a side note, let's mention pricing.
For the most part (and I know there are some minor exceptions), when a game debuts at a price higher than a couple of dollars, you can expect the price to be lowered on it eventually. Most likely after a month or less. This is fairly predictable, and I've seen a developer friend of mine doing a similar thing. I know that you can get graphs on purchase trends for your apps. After the initial buying rush subsides and you see the graph slope tending towards zero, you lower the price. Certain app monitoring sites and tools notice it. A site may even write about it. Then you'll see a second peak as effectively the last of your sales happen (those people who waited for the inevitable to happen or who were on the fence about buying the game in the first place). The only people who will pick up your app after that will pick it up when you make it free for a day.
I'm sure there's some statistical maths to be done here (by someone), but I'm guessing that debuting at a lower price (one that falls in the realm of impulse buy) might actually be beneficial. You would get more coverage and probably more word-of-mouth sales (and this is likely quite common given how many apps come out per day). Unless this is a massively hyped game that people have been itching to get for months. But those are few and far between.