Naive to think epic is offering a lower cut for altruistic reasons as opposed to it being the only method they could think of to try to convince devs to sell there.

This is literal the entire basis of our economy. A company being able to offer a service more efficiently charges less and gets more customers to come to them. It is the literal only mechanism in capitalist that keeps it running at all efficiently.

And that they wouldn't jack up the rate once they corned the market given how their how strategy has been more reminiscent of Walmart approach of pricing lowering to gain market share. Biggest sign is that the store isn't even profitable much like how lot of services these days aren't profitable and burn money then jack up prices and offer less once they corner the market. Hell even Microsoft Store has offered low rates of 12% because few want to use it. Going to argue Microsoft is nice too now? Not falling for it Tim.

How would they corner the market? Steam still exists. As you pointed out, the Microsoft store still exists. If they ever jack up their prices devs can go elsewhere.

No one is accusing Epic or Microsoft of altruism, they offer 12% because that's closer to what it actually costs them to run the store. Steam charges 30% because gamers refuse to buy games from anywhere else so they can just tack on an extra 18% more money that they'll take.