Originally, Steam's 30% cut was a huge windfall for developers (retail stores could take 60%+). But I won't harp on that, just a historical note for why people (including developers) might still like the (existing) middlemen.
The 30% figure is a bit deceiving. If you sell a lot on Steam, you can get a better cut from Valve, but you don't even need to sell a single copy to get a 100% cut - Valve charges nothing for Steam key generation. You could sell keys on your own website and only pay payment processing fees, or get a decent cut from Humble, GMG, etc and not have to manage a storefront. One-third of all Steam game registrations are key activations [0], so this isn't some insignificant revenue stream.
What HN gets annoyed with is exclusivity and lies. Valve never required exclusivity and did not try to become a monopoly (you could sell on GOG, Origin, and any other storefront alongside Steam if you wanted), they just sucked less than the competition (GFWL et al) and developed consumer loyalty through their sales.
Valve is far from perfect, but they're decent and aren't anti-consumer, so when Tim Sweeney starts handing out $2 million for exclusives [1] to undermine everyone else and lies through his teeth on Twitter, people get mad. Epic doesn't want fair competition or they'd focus on making a much better launcher and sell their platform on its merits rather than its exclusives. Tim doesn't care about introducing competition, because it already exists (GOG, Origin, UPlay, etc), and people are still happy to stick with Valve.
>That 30% is coming straight out of gamers' pockets
This implies gamers care about the cut like it's a tax, and that if Valve lowered their cut, games on Steam would be cheaper. I did a quick check and saw the new Watch Dogs: Legion can be preordered on EGS for $59.99. Logically speaking, if the 12% cut is priced at $60, buying from Ubisoft directly (where there's a 0% cut to EGS) should be even cheaper, but nope, it's the same price. The lower cut translates to more profits, and executives won't lower prices just because they're getting more money. The reason for this is that video games are intellectual property, not commodity goods where people can directly compare prices for equivalent products (that is, while I can pick and choose a generic paper towel brand over Bounty and get similar utility for less money, choosing Need for Speed over Forza will result in a very different experience, such that you can't really compare games on price).
Given that Tim wants to force everyone to use his crappy launcher with no intention of Linux support, I can see why HN would rally around Valve. Middlemen are necessary for distribution, server hosting, and dealing with scammers trying to chargeback thousands of transactions, so we might as well have a company that isn't actively hostile to consumers doing it. Valve could lower their cut, but it would do nothing unless they stooped to Tim's level and started paying for exclusives, and Gabe has made it clear he wants PC gaming to remain an open platform, making paid exclusives unlikely. Doing so would result in a race to the bottom, and Tim knows that although Epic has tons of VC money to fund such a race with, Valve's refusal to take VC funds over the years would result in them losing.
[0]: https://twitter.com/Mortiel/status/1120357602305560577
[1]: https://www.pcgamer.com/phoenix-points-epic-store-exclusivit...