1Password and Dashlane have announced support via web extensions. I also really hope we have future platform-level support, so that third party "passkey managers" will work outside of the browser (such as on mobile, within all native apps).
Wouldn't that completely eliminate the security benefits of passkey?
It really depends on your security requirements and where you're coming from or what you're comparing it to. There would be still many advantages over, say, passwords stored in a pw manager, such as no shared secrets across all sites/apps, phishing protection, and far fewer opportunities for user errors.
No, because it already uses a third party service, it just doesn't let you pick the one you want to use. I want to have the option to use a FIDO2 key, for example.