There is a reason they deactivated the accounts, but it doesn't justify removing the ability to migrate at any time. For those interested:

The old mojang accounts were not secure, and there were millions of accounts that could be accessed without email ownership. This created a grey market for cheap Minecraft accounts. These cheap accounts were almost exclusively used to cheat on non-cracked servers, which sucked for a lot of players who did competitive Minecraft games on servers. The migration did fix this problem, by requiring access to the original email or answers to the security questions. Migrating your mojang account also gave access to the windows 10 version of the game. It probably should have been allowed forever, and I have no clue why they didn't.