At the end, passkeys are built not for the tin-foil, (I hate Google Apple fellows), I want to keep every single locally, RMS fans. No.
A majority will benefit. End of matter.
A majority don't change platforms (I have not seen them do it).
And lets be honest - even if they were portable are you privacy person that is going to do it? No.
People break their phones. That is normal experience. Entirely unsupported by passkeys as they are today.
I'm actually surprised we didn't have more pushback for ubiquitous 2FA, as they have similar threat profile - i.e. addressing the tin-foil threats of cybersecurity aficionados, while entirely ignoring the common threats to real people, in particular the one of broken or lost mobile device.
People break their phones less often compared to telling them keep their keepassdatabase in sync across devices.
Even recently my friend fixed his iPhone XR (10 year old) 3rd party. Everything including passkeys work fine.