Reminds me of the "semantic web". Making content machine-readable has positive side effects. (clear structure, reduced ambiguity, separation of data and presentation)
It's another example where the reason for better engineering is to make machines (search engines) happy.
"Google-free" FOSS Android-builds (Graphene, /e/, iodé) are available today and usable for most tasks. Just make sure your government IDs and banking apps don't depend on proprietary Google-only features.
The dependency is much weaker in this case. Finding somebody else to manage/host Nextcloud is easy while using MS Office without Microsoft is impossible.
Web-apps in the browser can't be used for encryption because in that model the server is always trusted to send whatever code it wants. That defeats the point of end-to-end encryption. That's why Mailvelope is a browser add-on and webmail clients don't just embed openPGP.js. This way they can create releases of the crypto-code and distribute them over trustworthy channels.
If Isolated Web Apps (IWAs) take off, it may become an option.
The term is often used without a clear definition. As a passkey should be used instead of a password, it means that you need "user verification" (PIN or biometrics) to protect the access when the device is lost.
The other thing is that it is more convenient to have "discoverable credentials" (a.k.a. "resident keys") so the browser can check if there are passkeys stored on your FIDO2-authenticator for a specific website. (instead of asking for a username first)
Syncing is optional but if your passkey is not synced, it is usually called a "device-bound passkey" (e.g. Windows Hello).