I don't understand your complaint. I suspect you have a fundamental misunderstanding of identity management in practice, irrelevant of whether it's implemented with OpenID, OpenID Connect or SAML.
If you want people to register with anything, then just let them register with an arbitrary username and password. You don't even need an email or a domain.
I want what OpenID provided me: I give a site a URL to a page I control, and it goes there to figure out what service of my choosing to use to sign me in. I want this to be open, so I don't have to check with each site what whitelist of providers they accept.
I thus do not need to remember/manage a username/password pair per site, and am not tied to a specific service that might die, which was a big problem with the OpenID infrastructure dying bit by bit: those wo didn't use URLs they controlled to delegate lost their ability log in when the provider they used went away.
Most of the implementations were incomplete/broken, everyone wanted to provide OpenID while nobody wanted to consume it, and the few parties that did consume it got left holding the bag when the OPs ripped out support and left users without a way to prove their identity to recover their accounts.
You want at least a weak unilateral arrangement, such as Google or Microsoft or Facebook saying they have no plans to rip the feature out from under you any time soon.
Also, OpenID adoption was never going to happen with users being identified by URLs. No party was going to step up and explain to users why they should go through the effort of learning all this tech to use it vs. using the passwords they were already familiar with.
If you want people to register with anything, then just let them register with an arbitrary username and password. You don't even need an email or a domain.