1. Most applications will get "Passkey" support by dint of OIDC SSO support; OIDC IdPs are the things that will implement Passkeys (SIWA and SIWG for "retail" users).
2. Direct Passkey adoption in applications will round towards zero, maybe excepting huge applications like Insta; people will do Passkeys with their Google account, but not with (say) Doordash.
If those premises hold, I probably don't need to be sold (though this post is helpful and incredibly detailed) on why not to do my own direct implementation of Passkeys; it makes more sense for us to nail OIDC.