If side loading were possible:
1. Nothing forces you to download from elsewhere.
2. Nothing forces you to download from less than reputable sources if you choose to download from elsewhere.
Why is that so complicated to understand? Nothing happens to YOU personally, or your device, as a user, unless you were to make poor choices.
I don't understand why people make this argument. This is a non-argument. Even Apple tries to make this argument, and they know as well as anyone that it is flawed.
Apple simply can't say the truth, which is that it is 10,000% about not losing App Store revenue, period.