You don't have a choice, from Apple. With Amazon you can buy your Purina and flip-flops off a third-party, but where else do you buy a calculator app for iPhone?
The problem with this steelman is that giving third-parties more options should change nothing for you. If you're ideologically opposed to anyone that doesn't use Apple's or Amazon's fulfillment system, it doesn't matter where the competitors are anyways. In a post-sideloading world you'd keep using Apple's App Store the same way millions of Android users never enable Developer Mode. Both sides get what they want.