The iOS App Store design is pretty awesome actually. It got a total design revamp last year in iOS 11 [1], and it's coming to macOS Mojave in a couple of months, so to claim the design is stagnant is incorrect.
It's fluid, and has great use of typography, spacing and colors. I don't see how it is criticize the design of it, the design is the best part. The missing features (like demo versions not being possible) etc and bad policies are the issues, not the design.
An annoying thing with the new design is that the Today content only is saved for a week, past articles and selections gets lost (archiving the editorial content would actually be a smart website idea, if you disregard the copyright issues), ideally it should be indefinitely scrollable.
Edit: It's frustrating that people don't bother explaining what is wrong with the design of the App Store instead of just downvoting me.