F-Droid is even more locked down than the App Store, and even if they turned it into the default app store then Android would still be riddled with malware.

As to how the Android users keep acquiring malware, I have no idea whether it’s from the Play Store, or if they download free apk files of paid apps, or if they download it through ads or from emails or whatever. I just know I have to help fix them regularly, and if iOS is forced to open up then iPhones will suffer from the same malware issues that you see with OSX/Windows/Android.