It is. iPhones don’t suffer from these malware issues precisely because everything is locked down and you can’t get apps to the iPhone without Apple’s manual approval. If Android was locked down and only allowed apps to be installed through F-Droid then they wouldn’t have any malware issues either.
The reason F-Droid is safe is because they’re also locked down and their approval process is even more strict than Apple’s.
> I don't think anyone is arguing no store can be locked down if you have multiple choices.
What I’m saying is that once you allow third-party app installations then malware will follow. And as we can see with Macs, not even Apple will be able to prevent it no matter how much they try to discourage people from downloading apps outside the App Store. The malware you see on Macs are a result of users downloading apps outside the App Store, and the same thing would happen to iPhones if they allowed third-party app installations.