I don’t think any company would want to keep a separate auth system if they can help it, though. No one wants to spend resources maintaining redundant account systems and interfaces between them when they have the option to consolidate.

But I suppose I just don’t see why there had to be a deadline at all. They know the email addresses associated to the original accounts, so there should just be a database they can reference to check if that email address has an unmigrated account and prompt to convert when next they log in. This is beyond “I don’t want to maintain two account systems,” it’s “Let’s just throw away this old table of email addresses so we can make more room on our servers for telemetry data.”