The passkey is stored in the secure enclave which is protected by your unlock code & biometrics (for fast unlock), or whatever the equivalent is for Google. You're not replacing your secret key / passphrase, it's just a different way to authenticate and decrypt vaults.
It's really more a convenience on a handheld device where entering a passphrase (mine is 20 characters) is a pain in the arse. Though, if it were available for macOS, I would use it there too.
I feel that is too risky. Where would you store that passkey? And what if you lose it?
I suppose the 1p emergency kit could print out a QR code or something.