I would disagree about "always" polar opposites. For example SSO/iDP like Okta add a ton of security but also makes it easier while more secure for employees to log in to their multitude of company apps.
I think the catch is that people only notice when they're opposites. If making something more secure also improved the usability (or didn't harm it), then things would (generally) be done the better way. It's only when there's tension or compromises to be made between the requirements that we actually see it, and start to think that they're opposites - because in those cases it is.
While I agree that things like SSO can be more secure and more convenient for the user, that doesn’t actually make them more convenient overall. In your example it’s mainly the fact that Okta has abstracted away the complexity so that businesses don’t see it.
I’m open to being proven wrong, and as eythian implied, I may in fact be subconsciously cherry-picking times where security and convenience were opposites and missing times where they were not, but even with that awareness no counter examples are coming to mind.
