The original vision for OpenID was for users to own their identity. They could choose their identity provider (including self-hosted) by providing a URL. But it got distorted into Google/MS/FB/Apple owning your identity.
> there's nothing stopping me from automatically migrating every Google user to a local account pre-populated with their Google claims
I wonder your user retention would look like in that situation.
That's why you should always provide multiple method of authentication.
A simple way is fetching your user email from the OIDC provider, then use it for username/password authn. It's less convenient, but it prevents user from being locked out in case something's wrong with OIDC.
You answered your own question. This is an issue with the app creator not supporting multiple providers, not with OIDC.