No, you don't promise to pay anything back, you promise to give them a share of the profits, no profits means you don't have to pay anything. When you take a loan you promise to pay it back, profit sharing doesn't come with such a promise.

> The problem here is the agreement is uncapped, leading to things like a preference for additional profits over additional productivity.

It is uncapped because otherwise nobody would give you any money. If you want peoples money you need to give them a good reason to give it to you, and sharing a fraction of the profits you make is the only thing you can give them to make it worth it to them to give you money.

If you have a very stable business idea with no risks, then some people would be willing to give you a loan. But that doesn't cover most startups. Instead you have to sell a share of your profits.