Are you going to force people to use specific smartphones?
Even then, nothing keeps the vendors of alternative smartphone OSes from implementing a FIDO platform authenticator.
Those that fight the duopoly and allow user freedom: Librem 5 and Pinephone.
I really hope that it could use an open standard. Then, it's probably fine.
https://fidoalliance.org/fido2/
Nothing is preventing either OS from implementing it, either as a platform authenticator or via NFC, USB or Bluetooth support for external authenticators.