Instead, each website that accepts passkeys should allow you to register multiple devices and probably print out backup codes as well (for the especially important accounts).
If there's no reason to migrate anything then lock-in is irrelevant. Just add more login methods so that when you lose some, you have others.
With backup devices, whenever I upgrade or replace a device, I need to go to each of the 500+ online accounts and register the new device. This is much more work than a quick login to each site via my password manager (which can happen on-demand, only as I need to use the services).
See Show HN post I wrote this morning https://news.ycombinator.com/item?id=33178285
Websites already have a hard time to get users to sign up, so requiring them to enroll backup authenticators (which they won't have) is not going to work. Printing or writing down backup codes is even worse from a UX point of view.
IIRC the spec has a flag to hint that the passkey is backed up (in iCloud or your Google account) so the relying party (website) knows whether backups are mandatory but that means the secret doesn't stay on your device and goes to the mothership. Then I don't see why the spec wouldn't standardize the transfer of secrets from one company to the other.
It would be ideal to set up backup auth before you need it, but you could also do it when you decide to move off Google.
So, not really a lock. It's a vulnerability, though, if you lose access to the Google account for some reason.