A dude called Karl wrote a book that answers this.
In my libertarian-socialist days, I have actually gave some thought to this question. The best I could come up with was 1) variable return lending or 2) profit sharing agreements with a sunset duration.
Both seem like a more expensive option for companies in an environment with economic growth. The beauty about selling stock/corporate ownership is that you never have to pay your investors a dime as long as the company has growth prospects. The return for investors simply comes from sale to other investors.
When a company issues a bond or makes a profit sharing agreement, they actually have to pay that back. Maybe in a growth environment, they could perpetually use new loans to pay old debt, but this would put them at the mercy of the outside lending rate.
By way of example, instead YC buying 20% of a company, it could receive the rights to sell a 20-year contract for 20% of the company profit.
If AMZN is a $2T company and has 1.5M employees, every worker can become a millionaire if they just do this one simple trick. Why don't they?
I'm also confused about what role you think investors should play in business if money is unimportant. What would YCombinator look like? A bank that just loans out money for a fixed interest rate?