Steam is dominant. Which means that the other stores that are successful (meaning they make money) probably won't try to compete with Steam in rates, because they don't have comparable marketshare.
No, this shows a fundamental misunderstanding of how the market works.
Literally every other storefront competes with Steam on 'rates' very easily.
Anyone can sell Steamkeys directly from a games publisher, including the publisher themselves if you want to make sure that the entirety of the purchase price goes directly to the publisher. Or give a percentgae to charity with every purchase, as Humble does.
Steam does not make a penny from any sale not made directly through Steams storefront.
Conversely, MS - whose storefront you are championing as "competition" - is the sole distributor, supplier and vendor of
all UWA keys. It is entirely impossible to create a UWA without using MS made tools, going through MS terms and conditions, and going through MS certification for every release.
The only party that unquestionably benefits from UWA is MS.
By using Steamworks, any software company can create, update, and sell products without ever paying Valve.
e:
Let's not pretend these exact points of comparison that work wholly against MSs favour haven't been brought up a hundred times in UWA topics now either.