If sites started going Passkey only even for "guest" access, you'd quickly see a rise in Passkey managers that quickly generate throwaway passkeys and you'd als

If sites started going Passkey only even for "guest" access, you'd quickly see a rise in Passkey managers that quickly generate throwaway passkeys and you'd also see a rise in "globally shared" anonymous group passkeys, just as people already do today to get around paywalls and some walled gardens with passwords and "phone memberships" at rewards clubs (the notorious Jenny's Number being a common one there). The techniques don't really change that much. Passkeys really don't have any more identifying information than passwords or phone numbers and temporary anonymous passkeys or over-shared passkeys are likely to be a thing in some places as soon as they are seen as necessary.