But technically, you can download the source and/or write your own code, compile and install on your iOS device without jailbreaking or going through the AppStore. It's a hassle, sure, but doable.
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)
Depends on how you define noteworthy and large. The xcode backdoor thing was in 500.000.000 downloads.
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!
Security is becoming EXTREMELY important, although it was already pretty important.
Apples very strict (occasionally draconian) app model on iOS is one of the reasons it has so few problems with the software destroying the system or viruses or other such things. There's a reason Microsoft is eyeing it jealously for Windows. That kind of heavy sandboxing makes exploit much much more difficult.