I'm not sure what UX you are talking about, the majority of the websites supporting u2f/passkey have UX to manage your u2f keys/passkeys. (the only exception I can think of is early Twitter when it first implemented u2f, and at that point it only allow you to add a single u2f key, but even Twitter fixed that later and supports multiple keys now).
And (this is probably not emphasized enough) you really should never only use a single u2f key/passkey for a website, that's the recipe to get you locked out when you can't find your u2f key/get locked out of the provider of your passkey. I have at least 2 yubikeys on my keychain all the time (one for usb-a and one for usb-c), plus one for each of my computers, and passkeys from 1password, google, etc.. And whenever I add u2f keys/passkeys to a website I add all/most of them.
...and you just described why this is not ready for prime time. Managing a number of physical devices tied to completely opaque secrets stored by unclear providers in places you never see, with hidden agendas promoting their locked-in solution over all others and complicating everything out of one ecosystem.
Most standard users will either mess up royally or run away scared. Damn, I've been on this field for 30 years, I've been using 4 OSs, 5 different browsers and devices from every ecosystem, and I still find this whole thing too much of a hassle.
And yes, I do have a backup passkey. Even though I had to convince my skip-level that it made sense. I just find it all too complex to adopt it broadly.
if I am reading right, any time you set up passkeys on a web site, you add half-a-dozen passkeys from various services? Yeah, this sounds totally impractical to me.
Have you considered stopping using passkeys and using strong passwords stored in password manager instead? You will have approximately same level of security:
- Either way, if one site is compromised other sites are not affected (because password managers have site-per-password)
- Either way, you will be phishing-protected (because password managers autofill based on host name, and you are smart enough not to override it)
- Either way, it'll be game over if you get a malware on your computer (because it will steal your passkey out of 1password)
... but your UX for new website would be dramatically simpler.
It's not much of a hassle. I'll add at least two when I want to start using a passkeys for a site. So maybe add a passkeys to the phone and to my keychain device. Then, next time I use the service on my laptop, I'll sign in with either my phone or keyring (whatever happens to be closer) and make one there. Then next time I want to use the service on my desktop I'll sign in with whatever I've got nearby and add my desktop and the token in my desk drawer. And maybe my password manager also has a passkeys, added somewhere in there.
It's not like every time I sign up for a new site I have to drop everything right at that instant and go add a passkey to every single device I own.
And (this is probably not emphasized enough) you really should never only use a single u2f key/passkey for a website, that's the recipe to get you locked out when you can't find your u2f key/get locked out of the provider of your passkey. I have at least 2 yubikeys on my keychain all the time (one for usb-a and one for usb-c), plus one for each of my computers, and passkeys from 1password, google, etc.. And whenever I add u2f keys/passkeys to a website I add all/most of them.