I think we can rule out any backwards incompatible solutions such as removing the validation or changing the default to all valid schemes. I don't see a sufficient justification for breaking all sites relying on the current behavior. You'll need to argue your rationale on the DevelopersMailingList and get a consensus if you want that.
It seems to me that there are too many customization points on validators to add a new model field argument for all of them (such as allowed_schemes as you proposed). For example, there have also been requests to customize the validator's regular expressions. It doesn't seem clean to add a model field argument for each one of those validator attributes.
I'd like to try for a more general solution to allow customizing the default_validators as I described in #25594. I believe that would meet the use case of this ticket.