That or another iCloud device. If you have a couple, that can be your security backup (afaik).
I do think the calculus changes for yubikey. Without built in security keys, every knowledge worker on earth should have a yubikey like thing, so their market is huge. With built in device security, then the keys might not be deployed at the same rate.
It's a good point though. I also think companies (at least mine) like having full control over the yubikey experience whereas the way apple manages the secure enclave is more obtuse.