Developing a web application is a very twisty road. There are a number of assumptions that you'll be making about your market, the problem that market is facing, the solution you think can solve that problem, and the channels you'll be using to market your solution. Is it possible to pay someone to be on staff to execute your changing specs and meet your ever-changing use-cases? Sure. But it's going to cost you a lot of money. It'd be better to have someone who has a vision for solving the market's problem who has the technical chops to execute on the idea and to be flexible when your needs change.
Put another way: Launching the app is really only the very beginning of the process.