I've seen it. Websites are pretty inconsistent about the flow but I've had a few just try to install a passkey when I log in (amazon does it. Paypal seems to do it but only try to use it on the same browser it was installed on, even if it's synced through a password manager, just to be different. The more reasonably ones have a 'log in with passkey' button but this isn't universal. Microsoft seems to just nag the user on login periodically in their classic 'Yes/Ask me again later' style of 'consent'). You can, if you're paying attention, avoid it actually happening, but the average user isn't going to notice until they suddenly can't log in because they're on a different device. You can see some advocates are pretty vocal about making them mandatory as well.