I think this is a good point, even if you're presenting it as a false dichotomy.
Obviously saying "Apple shouldn't be allowed to touch my device after I purchase it" as well as "Apple should be compelled to provide security updates" is nuts.
But I think saying, "Apple shouldn't be allowed to touch my device after I purchase it" as well as "I should be able to provide my own security updates, if Apple doesn't want to" is totally reasonable.
But Apple would never allow that. So allowing sideloading seems like a reasonable amount of pain Apple should be forced to put up with...
I don't think Apple should be compelled to provide security updates. I think Apple should be held accountable for security vulnerabilities in anything they release. You can't evade liability by patching it later.