Just a little clickable logo in the bottom of the page would be quite enough really, and remember I am not directly comparing passkeys to username/passwords, but to username/passwords as one factor and one of the previously mentioned methods as another "factor" in the sort of 2FA that The_J mentioned. KeePassXC is easier for me a lot easier than both factors, and frequently counts as two.One need not literally offer both services to know about audiences reactions. The effect of increasing complexity on accessibility and user base size can easily be observed from any number of cases where this *was* done already.
In more general terms also, studies of consumer behaviours and actions allow for some reasonably quite well educated guesses as to whether making the registration process more complex would harm accessibility (and usage) of a service in the long run.
Even pushing Passkeys as the default option and requiring an extra step for people to chose an alternative could do that. For most online services, the risks taken versus potential benefits are just not there to make this a reasonable option to push.
How can one program count as two factors? I can list some different methods of 2FA:
Username/passwords: Everything does that, including KeePassXC.
TOTP, ie. Google / Microsoft authenticator: KeePassXC does this, and does not require you to go to big tech
Hardware based: If you want to buy for example a YubiKey KeePassXC integrates with that well
Out of band One time passwords, ie. SMS/email link: That depends on the definitions really, but it should not be able to do this. Off course, that frequently means going through big tech, which considering the current climate I am not sure is a great idea to be driving people towards.
Which two of these would be really easier than one well supported open source tool, that came easily through the GUI install tool on my system?