I solve this by creating two passkeys for each account, for iOS and Chrome. This covers all the devices I might use to log in and there’s no single point of failure.
Even passwords are better. Seriously.
I'm honestly already glad that they didn't make their annoying browser fingerprinting rigorous enough to also block authentications and aren't requiring attestation.
Besides passkeys, some websites have alternative backup authentication, though, such as backup codes. However you do it, there should be an independent way to log in.
Regular users would not set up double passkeys, and would suffer vendor lock. They would be better off with password manager instead.
Savvy users know how to follow password hygiene, and have no need to have it enforced on them. So they don't need passkeys either, they would be better of with good password manager.
More generally, you should have two keys for any lock, so you can still get in if you lose one of them.
* Lost access to your Google account, but still have access to your Apple account (or vice-versa)
* Apple device doesn't support Google passkeys, or vice-versa
But it multiplies the following downsides:
* Apple/Google account gets hacked, hacker gets all your 2FA credentials
* Snooping on your activity. Particularly Google, but Apple also have an advertising business.
* Setting up accounts on new sites is twice the hassle.
* Too complicated for the kind of folks who need the phishing protection passkeys provide.
This just seems to mean that, instead of being the whim of one corporation from being locked out of all your accounts, you're the whim of two corporations away. That's better, but I'd hardly call it solved.
(If it ever happened, Gmail and Google Photos would be a bigger loss.)