This is why timetravel is, in its very essence, evil.
Specifically timetraveling to the past.
The 'now' is already happening. By going to the 'past' you murder the current 'nows' iteration. Scottsman, Ashi, Eemakandi, Moth-prince and Moth-princess, The Dogs, The Ravers, The Elephant things, Jump-Good Monkeys, SAM-UH-RAI, X-9, LuLu, That wannabe Ghibli Monster thing, The Archers, the Spartans, the Mobsters, the Sea Men, all those other good characters are obliterated for the sake of Jack going to the past.
So... most writers pull the bullshit of "oh they still somehow exist in the future anyways after a major force like Aku is removed, but HAPPIER", or they'll do what is, all things considered, morally correct and Jack will stay in the future.
Good or bad though I want to see what Gennedy pulls off. He's DONE time travel before. Lookin' at Dexter's Lab