. So if you're running your own store, you'll spend 20-25% on payments and marketing to sell a quarter (at best) of what you sell on Steam for 30%. Steam knows it and most developers know it too.
Problems with causality in your post. Steam is taking 30% because they have a monopoly and can change that number at will, all
your reasoning is just fitting, but it's not the actual reason why they're taking 30%. It's not necessary to steal 30% from devs, for proof: see Epic. Don't rationalize or romanticize thievery. You're too in love with exploiters of other people's labor. Which, for an indie dev, is being a useful idiot.
There's two sides to this story. Steam lets indies sell games and make profit woohoo. Steam is a thieving monopolistic exploiter, not woohoo. Ignoring one of two is being a cuck.