I'm not aware of many cases where the SSO registration page is turned off after it has been enabled. Typically it is turned on because there are pieces of data not available with the customer's IDP such as login (they don't have a username that is appropriate for forums), missing emails, or as mentioned previously community specific terms of use.
Note that once you disable the SSO registration page, any fields that were previously managed by the community will then need to be handled by your IDP. In that case, it may not make sense to have a 'registration success' page in the community as it should be a seamless experience at that point. I am not aware of a feature for batch updating partially registered users with random, uniquely generated usernames. One option may be to use the user export admin tool to help with that process or you can engage Professional Services.
Another option is delaying display of the SSO registration page until a user performs an action that requires a username (such as message posting), but I think you may already be using this in your community