This is slightly revisionist. The App Store is really just a centralized portal for software purchase and download. People were purchasing software online and downloading it well before the App Store.
IIRC, it had become the standard for high-end, engineering, server, unix software by around 2002, but App Store really did do a pretty good job of replacing boxed software in the $20-300 range outside gaming (which Steam probably did first).
You mean "App Store did a really good job copying a standard practice in the software world to a specific domain that Steam was doing a really good job in already, and though brand and lock-in, managed to survive where on pure merit, it wouldn't have".
No, Steam was never doing a good job of selling apps outside gaming, and still doesn't.