Step 1. Sign in using your existing device
Step 2. Add your new device
Step 3. Remove your old device (or keep it for a backup)
And if you feel that's going to be a big issue for you, use a 3rd party software based tool like Bitwarden, 1Password, or other tools that have indicated they'll be supporting this. That info will sync between those software based tools and allow you to use whichever devices you wish, across multiple platforms with minimal effort.
That's possible, unless you are switching device because your previous one broke (or was destroyed/stolen/lost).
I imagine that Apple has very precise data about the reasons why (and situations where) people switch from iOS to Android, and making that switch as unthinkable as possible is part of their user-retention efforts.
This is my general stance on it as well, and one that I think I would still strongly recommend even in this new Passkeys era. That would cover situations where your device is damaged and you can't easily turn the old device on.
Part of this is solved by these keys being synced on each's services, they're synced via iCloud Keychain on Apple's side for instance, so you'd just need to get a new Apple device, sign into your iCloud account and provide the password for iCloud Keychain then you're off to the races. But if in the middle of this you opt to switch from Apple to Android, you're SOL unless you have a backup.
Just my two cents on it, I don't think any of this completely solves the need for backups, but for some portion of people it probably does, and that's those that are unlikely to switch between platforms.
