I feel like if they did do Ancient History Star Wars, it would end up like Enterprise in Star Trek. Namely, that the big draw is being in the past when all the familiar signature races/tech of the series haven't been discovered yet...but then said signature races/tech are immediately discovered and the original distinction becomes meaningless.

Going 'back' instead of 'forward' does have its merits in general. The question is if 400 years ago is far enough back to allow creative storytelling freedom without being constrained to existing canon. I don't want a Star Wars version of the Fantastic Beasts franchise where it becomes a parade of 'remember this old character from the original movie? See them now as an adult/kid!'. Middle-Aged Yoda fighting Darth Plagueis while Young Sideous watches is not something I'll want to see.