Why the hell was this comment downvoted? It has correctly summarized the problem: the company is effectively selling umbrellas for 1/3 the wholesale price. Increase the deposit to 70 yuan and you don't have to mind if people keep the umbrellas.
The reason it failed is because of this simple use case:
You want to return it just after you arrived, and it is still raining outside (this is why you rent an umbrella in the first place). If you wait for the sky to be clear to go out, you won't remember the umbrella. But if it is raining again when you go out, why would you want to return the umbrella???