It's a nice thought, but overall computer literacy is still highly varied, and it likely will be for a very long time.
We still have a large percentage of users who use computers sparingly and by rote. I have family members who need a lot of help to do day to day setups and are going to have a hard time with MFA devices or apps.
"Other people could have an old phone registered as well if they don't want to buy a security key. It's a very minor hassle to set up and I can't see why people can't do it."
Minor hassle for you. Major hassle for a lot of users. Try real hard to put yourself in the place of a 77-year-old user who has limited sight and only needs to use a computer to accomplish very specific tasks - and has zero interest in doing more than basic email, banking, and a few other things that can only be done online. They have a smartphone only because it's a connection to their grandkids.
Because of the smartphone they're saddled with a Google or Apple ID that they'd otherwise never bother with. A TOTP app or YubiKey? That's well outside their comfort zone.
This isn't because these users are dumb. But the assumption that "it's time everyone learned" is based on the idea that everybody is using computers regularly and has resources for educating them - which is simply not true.
My kids, my wife, and my in-laws all use computers very differently than I do and it's extremely educational how people outside the industry see and use computers.
My 17-year-old only uses a Chromebook for school (grudgingly) and would rather do everything on their phone. My wife is fairly computer savvy, but still hits roadblocks. (She does enjoy forwarding me screenshots of particularly bad Phishing attempts...) And my older in-laws occupy most of their time far, far away from their computer. Singular.
Anyway - it'd be lovely if folks had way more empathy for the huge swaths of people who have less experience with computers. It's not the priority for them that you imagine that it should be.
