For it to “even out” they’d only have to increase your reach ~50%.
They do way more than that. And they give you an inherent legitimacy that putting it on your own site doesn’t. It’s not just handling refunds; it’s the certainty as an end user that you’ll get one hassle free.
Without Steam (or another retailer with similar traits), selling an indie game would be closer to a pipe dream than really hard. In almost all cases (and this seems to apply even to AAA publishers as most of them come back), the 30% they’re taking is money you wouldn’t have without them.
I think there are a lot of people who weren’t around for, or don’t remember, how buying digital titles was before Steam got quite so popular.
It was pretty rare, and the overwhelming majority of indie games were released for free. There just wasn’t many good ways to get the word out, and most ways of taking payment were costly enough to set up that it was rarely worth trying to get some meager amount of pay if you were just a one man show with no external financial backing.
And exactly none of that matters because Valve has never attempted to maliciously take market share. If someone else wants to step in all they have to do is stop being shit. Steam has tons of issues. From the limited UI adaptability for devs to the rather archaic games list and somewhat silly discussions forums from the 90s, all the way to the convoluted larger menu system.
Yet rather than put any real effort into things we get shitty launchers from 9 different companies ONLY selling their limited scope of bullshit.
But they do give you an advantage. If steam didn’t exist at all, without a comparable replacement, it would not be possible for you to move a real quantity of units at all. The market they provide has massive value, and their market share is a product of genuinely being far and away better than any alternative.
People don’t refuse to buy games on Epic or Origin or Uplay just because they need everything in one place. It’s because all of those platforms are so much worse that they degrade the experience of games purchased through them.