You realise that your app idea had better be totally revolutionary?
Noone has said it in this thread, but it is very rare that an independent
developer makes the kind of money out of an app that would return on $20,000.
A developer doesn't need to make their money back on their first app. They can make $0 on their first 9 apps, and $20k+ on their 10th app, or average a few $100 per app over a few dozen apps, etc. Then start profiting on their 2nd dozen apps. There are also indirect monetary returns (app visibility leading to $$$ consulting contracts & etc.)
Second, an app doesn't need to be revolutionary. It can be just a decent app, but with licensing from a high-profile brand, or associated with a big marketing campaign or a viral hit on youtube, or targeted at some (rich) specialty niche or business which is underserved by all the devs going after the bigger more generic market segments. Etc.
Furthermore, there are a good number of totally revolutionary-looking apps languishing in the bottom of the app store due to lack of any marketing.