I promise you there is a significant percentage of people that would fumble enrollment; you handwaved away a giant problem (multiple enrollment, not present); and many people would put them all on the same keychain.
In the politest way possible, I question whether you've interacted with the modal user.
edit: I can try to dig up the article, but here's the precis: 5-ish years ago, google briefly changed their search results ranking. Lots of people were logging into facebook by searching facebook, instead of typing facebook.com, then following the top result. Some other site briefly was the top result when searching for google. That site got a wave of users submitting help requests because they couldn't log in with their facebook credentials, and accusations of subterfuge or wrongdoing because their accounts were deleted. I think it was pinterest, but I may not remember correctly. Either way, it looked nothing like facebook and didn't use blue.
That's what a significant fraction of internet users are like.
In the politest way possible, I question whether you've interacted with the modal user.
edit: I can try to dig up the article, but here's the precis: 5-ish years ago, google briefly changed their search results ranking. Lots of people were logging into facebook by searching facebook, instead of typing facebook.com, then following the top result. Some other site briefly was the top result when searching for google. That site got a wave of users submitting help requests because they couldn't log in with their facebook credentials, and accusations of subterfuge or wrongdoing because their accounts were deleted. I think it was pinterest, but I may not remember correctly. Either way, it looked nothing like facebook and didn't use blue.
That's what a significant fraction of internet users are like.