The comparison to knives, whose function and danger is completely obvious by inspection, is not really apt. Compare it to a home furnace or water boiler instead. These are regulated, both from the state and from independent testing labs. You can make and install your own if you know what you're doing, but no reputable installer or contractor will even consider installing some jury-rigged non-compliant contraption.
A checkbox that says "I understand the risks involved" is never going to cut it, because there is 1) no way that the overwhelming majority of users understand the risk, and 2) the existence of the option puts us in a world where completely naive users are told "to install the cat-picture-a-day app, first click the 'allow sideloading' button and ignore the warnings, and then go to cat-picture-a-day.ru and install!"
As the article notes, we saw this with malware and browser toolbars in the 90s, and we see it now with misbehaving sideloaded Android apps (and "repurposed" Android apps). I personally am happy to pay the premium to get a quality device that forces developers into a constrained environment for my benefit.