I would answer your question this way: accepting the definition of “stupidity” I cited above as sufficiently justified, we no longer want to call anyone stupid. After all, it turns out that this someone is stupid only insofar as we have measured it against our own standards.
After some reflection, I’d add that my opinion is still incomplete. It doesn’t take into account cases (for example) where someone asserts A in their first sentence, and then not-A in the next, and is completely oblivious to the contradictions. We also call this behavior “stupid,” but perhaps other suitable terms could be found: lack of education or cognitive errors…
Oh, these endless clarifications =)