So Google already allows sideloading. There should be no impact on them.<p>Apple allows sideloading today through their Enterprise system that allows businesses to create their own software that is signed with their own keys and then distributed through their own limited internal App Store. And attackers have already successfully abused tha5 system for malware.<p>So, if forced to allow sideloading more widely, Apple is absolutely right about what will happen with more malware.
Apple also has previous experience in this field with Cydia and the like. Again, lots of malware was found hiding on those alternative jailbreak sites.