noteworthy large outbreak of malware
Depends on how you define noteworthy and large. The xcode backdoor thing was in 500.000.000 downloads.
The xcode backdoor thing could have been much worse if iOS allowed sideloading for regular users. With the centralized appstore, most users having installed backdoored builds would be auto-updated to clean builds, current backdoored builds would be removed from sale, and new backdoored builds are likely to be blocked during uploads to itunesconnect/app review.
Also, don't forget the whole xcode backdoor fiasco happened because people were installing side-loaded xcode distributions instead of getting it from the mac store. So even where sideloading is and should remain allowed - on the mac - people will click big red "I know what I'm doing" buttons without knowing what they're doing. And that's for people who really should know what they're doing - developers!
Of course, it also makes good business sense for both Apple and third party developers as it keeps piracy to a minimum (though I'm sure a subset of 3rd party developers might find skipping the 30% Apple cut valuable)