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Implementing ATB would be a monumental change to the battle mechanics, the biggest the series ever had by far.
It would force them to retire the priority bracket as it currently exists, basic moves like Quick Attack would need to be reworked and certain abilities, if Legends even has abilities, would need to be overhauled.
It's a frightening prospect because Game Freak isn't exactly known for giving new gameplay mechanics proper thought. Just look at the mess that was Little Town Hero.