Having watched "The Founder", when Ray Croc was selling ice-cream machines nobody wanted he was listening to motivational records where the speaker says "the world is full of educated fools" and highlights that the one quality required is persistence.
Of course, if it's a stupid idea or a useless app then persistence is also useless and it would be a true waste of time.
But if the app is solving a problem for you, it likely will solve a problem for someone else which is why a side project will always prevail with persistence.
I always think that a side project is still a project, much like your main job. The only difference is time spent on it. But it'll be the same outcome.