1. Most products have a short self life. Think of all the iPhone apps. In 5 years we will probably be on the 3rd version and there will be lots of new features. If the application critical it is likely that the feature will be built into the iPhone at a later date or a free version will come out. If it isn't then it will reach market saturation and then the sales will die off.
2. You can make the sales increase by releasing a new version and fixing bugs. Depending on the application this can extend it's life by a number of years. You will also need to learn how to market your application. Do you want to spend time learning about internet marketing, seo, mailing lists, adwords, etc? Some people find it fun. Other people hate it.
3. Think of all the applications that you were using 5 or 10 years ago. How many are you currently using that you paid for?
4. From what I've seen there are two different ways that you can try to make money. Either make a product that people really need NOW and try to hit a home run and make a couple of hundred thousand dollars and if possible sell it to someone else before you reach market saturation. Or try to make try to link the product to a service. If you can make a service and make $10 profit then you only need 500 subscribers and you'll make a good living.