Eh, kinda a weak argument. Too easy to counter with "but sideloading would let that happen more!" That might even be right, and a difference in amount is important. There will never be a totally secure system, after all.

I think the actual problem is with how the App Store changes the way people think about and relate to software. The fact is, running code on your computer is dangerous. You are trusting it with control over its operations. The responsible thing to do is provide platform-level safeguards (permissions systems, sandboxing) and engender a general understanding that you should only run an app vetted by someone you would hand your phone to.

This is fundamentally incompatible with software as a market, of course, so this path will never be taken.