In the scenario I described you're doing that only after the user has explicitly opted in on a case by case basis, and you're forcing a per-session nag on them in order to coerce mainstream website operators to adopt the secure default.

At that point it's functionally slightly more obtuse than adding an exception for a certificate (because those are persistent). Rejecting the latter on the basis of security is adopting a position that no amount of user discretion is acceptable. At least personally I'm comfortable disagreeing with that.

More generally, I support secure defaults but almost invariably disagree with disallowing users to shoot themselves in the foot. As an example, I expect a stern warning if I attempt to uninstall my kernel but I also expect the software on my device to do exactly what I tell it to 100% of the time regardless of what the developers might have thought was best for me.