Indeed, when big publishers like EA and Ubisoft started leaving Steam they introduced a tiered pricing system which progressively reduces the cut to 25% or 20% after tens of millions of dollars in revenue, to lure those AAA juggernauts back. The price is now indirectly based on how much leverage you have over Valve - Ubisoft can get away with not releasing their games on Steam, so they pay 20%, while small-to-medium studios effectively have no choice, so they pay 30%.
It's especially backwards when you consider that those AAA games put far more strain on Steams infrastructure with their >150GB install sizes.
Heck, I've not bought games because they were not on Steam or required another launcher. Ubisoft and Rockstar are so bad that I held off on buying some games I really wanted to play; they're just that awful. EA's Origin was also pretty bad last time I checked.
I guess it's an actually hard problem to make a somewhat decent launcher in big companies with too many PMs playing turfwars, but still, almost everyone except Valve is shitting the bed so hard that as a consumer I'd happily pay quite the markup if it would allow me to avoid other launchers. They're that bad.
There are even games you can buy on one service and play multiplayer with people who buy it on steam! I chose to buy MSFS2020 through steam for example because the steam platform is dramatically better than the absurd way the Windows Store does anything, but we fly in the same skies!
There's no lock in or exclusivity. You can literally buy the same exact executable from multiple places, and the only change is the feature the store program supports. Buying a game through the Epic Store for example won't let you use steam input, but you can even then play it on the steam deck with some effort! I think you can even use Proton on executables you don't get through steam!
A dev can even make it so that, if you buy their game on steam, you do not have to have steam running or installed to play it. They have that freedom. They also have the freedom to mark a version of the game such that steam allows you to access that old version forever
If you are a dev who releases a game on steam, you can mint a bulk quantity of steam keys and sell or distribute those outside of steam!. Probably if you abused it, Valve would tighten it up or ban you, but why would you bite the hand that feeds you? It's how, for example, Humble Bundle initially worked.
That's right, you don't even need to buy your game from Valve to use all their features! A substantial portion of my library paid money to Amazon instead, through humble bundle.
People use Steam because it has 20 years of established trustworthiness in an industry otherwise made up entirely of assholes who hate you.
Meanwhile, in the place that Steam does poorly: Old games, GOG has much more of the market.
People actually are willing to pay for trust and care. Steam has repeatedly and regularly improved how their storefront displays information and informs consumers, because their primary problem is discoverability and wading through the mountains of games from people desperate to collect some of the money waterfall that Valve enables.
When you put a game on Steam, the contract ensures that anyone who purchases it cannot lose access without it being Valve's decision. Developers or publishers who do stupid things or pull games five years down the line cannot prevent you from playing a game you buy on steam if it isn't dependent on some server somewhere. None of the other storefronts have ANYTHING like this, mostly because they are run by the exact companies who WANT to be able to prevent you from ever playing an old game again, so they can sell the same thing to you in a new box.
Compare that to Apple's 30%, which similarly has lots of features their platform enables including unlocking significant consumer spending, but they do not give you any alternative. If you want even a single dollar from someone on an iPhone, you HAVE to pay apple 30%, and at least for a while they wanted that even to cover netflix subscriptions for example.
If you as a developer do not want to pay valve 30%, you are free to do like Notch did for Minecraft and distribute it yourself, and you are free to run into the same problem it had where my friend was unable to purchase minecraft for decades because his bank refused to send money to the Scandinavian bank involved, whereas even a literal child without a debit card can use birthday money to buy a steam gift card and purchase your game with no adult involvement. (maybe that's not a good thing for society, but it's great for game dev business).
Valve does not have a moat other than simply consumer trust. Minecraft sold a hundred million copies through a dude's website. There has literally never been a moat in computer game distribution. An entire industry of British children existed writing games and selling them in local stores. A moat has never been possible, because Valve cannot make your computer not run other software.
