The entire ending is equivalent to if Jack had killed Aku in their very first encounter in episode 1
Ending the story like this would be logically identical to Jack killing Aku in the very first episode. By all rights, Jack should not even get to keep his memories of his future travels since it likewise creates a time paradox since the subject of those memories (his adventures in the future) no longer exists. So without memories of his travels in the future, without Ashi, without any of the people or events of Samurai Jack S1E2 through the first half of S5E10 ever having taken place, we might as well have had Jack just kill Aku in their very first encounter. If we allow the plothole of Jack keeping his memories, then the story would be even more laughable: Aku basically put Jack to sleep for 5 seconds during which he dreamt up an entire storyline of being sent to the future, before waking up and killing Aku. This would be an “it was all a dream lol” ending. This cheapens the entire experience of Samurai Jack and invalidates all the obstacles Jack overcame and all the bonds he formed with the future characters during his time in the future. This is a big “lol fuck you” to all his fans.
Ending was extremely and obviously contrived to get the “bittersweet” ending instead of letting it play out organically
Ashi was reduced to a cheap tool of narrative convenience
This is perhaps my strongest point. It is so goddamn obvious that Genndy wanted to do a “bittersweet” ending before he even decided how he wanted the story to actually end. First and foremost, he wanted that emotional impact of a bittersweet ending, but the result was a very cheap suckerpunch to the audience’s collective faces; logical consistency, good writing, and pacing be damned. There are two very strong pieces of evidence for this:
1. Ashi suddenly knowing how to use a time portal
2. Ashi conveniently disappearing at a time that should make the greatest emotional impact