TBH this isn't a great argument for open source code. You know it's not doing something stupid in the exact same way you know a human written application isn't doing something stupid.
1- You review it yourself to double check OR
2- You hope that the community is reviewing it and that you would be made aware of problems OR
3- You just don't know.
I don't even think capitalism requires infinite growth. It's just how we built it. Not even since the beginning. That is, you could buy stock in a company to help them grow. Then they make a profit, and give you a share of that profit. Everyone is happy. You could sell that to someone else, and maybe they pay more than you'd get in a year, but they'll make more in a long run as long as the company stays alive and can keep distributing profits. Everyone is happy.
It's this idea that the money you make from investment should grow exponentially. This demand from professional stock traders that they be able to sell for obscene profits. The company must grow, and those profits must grow, or the shareholders will all sell in a panic and abandon them, and even a profitable company may go under.
Like why can't the company just make some profit and distribute that profit among shareholders and employees and everyone be happy? It doesn't HAVE to be more profit next year than last year, we just made it that way over time.