I've wasted so much time in these threads now. Wish I could jump back in time a few months and spend it doing something useful, like creative masturbation or something.
Look, guys. The real plot hole here is that Jack himself didn't disappear, along with Ashi, at the moment he killed Aku. The Jack that killed Aku could never have existed, either.
On top of that, he wouldn't even have any memories of Ashi even if he somehow just popped into existence.>Aku shows up> young Jack gets thrown into the future> shortly thereafter old Jack shows up and kills Aku
As far as anyone in the original young Jack timeline is concerned, young Jack just vanished into Aku's time hole. Then someone showed up and killed Aku, and promptly vanished. That older Jack doesn't exist anymore than Ashi.
Somewhere in the future, young Jack falls out of a time hole into a world where Aku was killed centuries earlier, but the world got on with itself and is nothing like what we saw in all the Samurai Jack episodes. Young Jack has no means to travel back in time, and probably doesn't even have the motivation to make the attempt anymore .. he just adjusts and figures out how to exist from the moment he appears in the future. Basically, he is the equivalent of a stasis sleeper, a one-way trip into the future.
The old Jack that killed Aku never existed. Nobody who ever met him ever existed.
The conclusion of the series SHOULD have been everyone vanishes at the moment of Aku's death. Scene fade to young Jack falling out of a time hole in the future, going "What the fuck?"
The fact that old Jack exists in the past, with his skills & memories intact, is evidence that another season (or more) is possible. If clever enough writers can be found to stitch it all together, probably requiring alternate universes.
Opening scene, season six: old Jack now hops dimensions, trying to find ways to kill as many Akus as possible, and save as many Jacks & Ashis as possible.