Even though Apple (and likely a couple more) will provide their proprietary passwordless system with cloud sync, a lot of people don't want to rely on an iCloud account to be your backup in case you lose your phone. Which means that cross platform password managers are in an excellent position to compete in passwordless, since it can span all your devices and browsers. They are already keeping sensitive secrets, integrate with biometrics and device-specific soft-locks, audit logs etc – all they need to do is add some integrations for WebAuthn and certain platform APIs that Apple and friends have to provide.
The main adoption-risk with password-less is fallback for when the user's browser isn't fully integrated with a pw-manager. Not that it's impossible, but if you need a backup you now have a multimodal and more complicated system, and if that burden is shifted to individual web/app developers it will certainly delay or even stop the transition.