In a competitive market, if your customers don't think your products do them good, they won't pay for them, so you won't be profitable. If they don't think they do them
much good, they won't pay
much for them, so you'll be less profitable.
Of course this isn't perfect --- they might be wrong about whether you're doing them good, like Lucky Strikes, and you might have stolen your products or dumped toxic waste or something. And there are lots of principal-agent problems, where the person making the buying decision has no incentive to care whether it turns out to be a good one. And the wealthy get a disproportionate vote. But it's probably a better criterion for whether you're doing good than anything else we can evaluate in real time in practice.
Although getting bank loans is pretty important, and getting wealthy angel investors is essential for some businesses, you can totally get rich by bootstrapping and selling to poor people. No already wealthy folks need be involved --- except where those wealthy folks are politically powerful, but that's a question external to capitalism.
Procter & Gamble sold soap and candles. Walmart sells to almost nobody but poor people. ExxonMobil started out, roughly speaking, as Standard Oil, whose main product was originally kerosene used for lighting and heating people's homes, and now is gasoline, used by everyday people to drive their cars around. (Rockefeller was no exemplar of capitalism, of course; he spent most of his life trying to destroy it because it wasn't profitable enough.) Berkshire Hathaway is generally considered to be the paragon of modern capitalism; it started out as a cotton textiles company, and nowadays its businesses are mostly things like GEICO (car insurance), Dairy Queen (fast food), Fruit of the Loom (cheap underwear), Acme Building Brands (bricks), Shaw Industries (cheap carpet), and Flying J (truck stops). If all wealthy people stopped buying from all of these businesses at once, they'd hardly notice.
It's certainly true, though, that the possibility of buying shares in new businesses reduces the opposition to upstarts. But public ownership is by no means fundamental to capitalism!