I would argue that an App Store, on mobile has helped avoid virus, and malware on phones, unlike how Windows was able to contain it. Having a trusted App Store by a company, who understands security, and has a financial backing to commit to a secure system, is beneficial all the way down.
The App Store is an app delivery mechanism. The same application sandboxing, API access restrictions etc. can all apply even if they allow sideloading. In fact that's already possible today through their enterprise deployment program.
Yeah, an App Store is in no way an artificial need. There’s a reason fortnite is on the Google Play store even when they had the option of only installing through other methods.
For non-savvy users it mostly solves the “evaluating installer maliciousness” problem. For all users it enhances convenience (unified method for installing and uninstalling software) and software discovery. It is not a perfect solution to any of the above problems, but IMO it’s very good.
In the absence of Google Play, uninstalling software would be in the same place -> settings / apps / needless tap to see all the apps / the app / uninstall. Installing would be download the apk and open it which works today, although you generally have to set a setting somewhere.
There's certainly a discovery benefit to Google Play, but anybody could build an app discovery site and make an app from that with an embedded browser, open the downloaded apks with the system intent and get the permissions prompt etc. You don't even need the special app management permission if you embrace the system tools that are already there. You would lose automatic updates without that though (but then, a lot of people in India have auto-updates off to conserve bandwidth or storage space).
Google play certainly does some small curation function, but I don't know how useful that is, and you could still have Google Play services running background scans and malware blacklists.
And believe it or not a Store does not have much to do with covering your ass when it comes to malware. There is the same security check on mac regardless where the application comes from (its done at the OS level) The only thing the App Store does is verify the developer because they have to be resisted with Apple (and makes sure Apple gets that 30 cut). Mac already does that btw (you actually cant download an app from an unknown developer by default) unless you choose to in the settings.
