The reason all games do this is because it's a system that both makes sense, and works.
If you already have enough money for what you need, meaning you start the game with money, or you receive money by intervals from doing nothing, what will stop people from making multiple accounts to farm gold? If money is not earned through missions, why would you do missions? for experience. That's like making our ninjas like a non-profit organization. Which we all know is not how it should be. Definitely though, we will have very rich ninjas, and things only they can afford. But the way they get this money is going to be from working hard and finding rare loot. It won't be hard to have a list of the richest.
But rankings aren't very important for now
Thanks for the suggestions!