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Passkeys Explained: The End of Passwords
(sentientrant.com)
This is a most excellent place for technology news and articles.
The assumption is that the native passkey manager on the device (iPhone, android, windows) would sync the passkeys (to Apple , Google, Microsoft) for protection against device failure and easy of use across devices. Or you risk loosing your accounts if you loose your device.
That would happen if you store your passwords there too...
If you're proactive enough with your passwords to manually store them in your own vault, you can be proactive enough to not use the corporate vaults that don't allow exporting. This isn't a "downside" of passkeys, it's a downside of using the built in managers.