Part of the concern is the friction of setting up on the order 100 accounts on an order of a dozen different devices over the next few year (I hadn't thought of that until you mentioned it), but its mostly that I don't trust any of the authentication providers to not accidentally lock me out of my own account.

Anyway, the immediate concrete use case for me is the same as being able to back up my password list:

If I lose all my devices (or, more likely, some cloud provider does what Google Drive did last week), then I can restore from the backup. Also, I've already arranged to be able to backup my password list and to store the backup in an offline / secure way. I don't want to set up a second account recovery path just for passkeys.

Finally, there's no data on any of my portable devices that I care about. I break a laptop or phone every N years, and I learned long ago to treat them as disposable. As currently implemented, passkeys break that invariant.