If you lost access to the email and forgot the password you can't demonstrate to the support person that you are who you say you are. If support people changed your password under those conditions then the system could be abused to reset random people's passwords. Also you don't know if your username/password still worked, if those credentials were leaked anyone could have changed your password, and Minecraft old auth system leaked a few username/passwords, I remember having a list of them that I used to introduce people to the game (obviously I never changed anyone's password, but there are plenty of douchebags out there)

BTW I never said this isn't an issue, IMO they should keep a DB and allow the migration indefinitely, so it is shitty of them to put a time limit on it. But if you lost access to the email for that account I can understand them not allowing the migration, part of activating an account is usually validating the email address, so whoever owns your email address now gets to own that account.