I'm the creator of Complice and it was indeed made largely by me (I have another collaborator but he joined after it was already making over $3k/mo) but I wouldn't call it simple! Both the software itself and in relation to your comment above that describes how it's not just "how easy is it to build?" but "how easy is it to market?"

In a world where most people are just looking for a better to-do list, Complice is offering an entirely different philosophy of productivity, so it can be hard to explain it, as opposed to something that just straightforwardly scratches somebody's itch.

I will say that the software plays to my strengths: I'm not a data structures & algorithms guy, or a machine learning guy, or a fanatic about the perfect REST API, and Complice is much more about the workflow and the interface, which is what I'm good at.

So "super simple" also depends on what you're good at!