This ending is just a tough call, and a hard ending to make. I initially felt unsatisfied after the ending, but I gave it a lot of hard thought and thought of how I would end it, and came to the conclusion that this is the unknowingly selfless ending. In my opinion, there are truly only two good endings given the paths the writers chose: the ending that we saw, or have Jack stay in the future with some unforeseen complication...this is where I initally rambled on about Jack's possible endings, not seeing a clearly better ending imo. See my edit.

Only things I wanted that weren't there: A longer 1v1 of Jack vs Aku, Ashi to live, if she were to die then a more impactful goodbye, I.e, Jack telling Ashi his real name before she disappeared or Ashi learning it off screen and saying it as part of her final words to Jack, more episodes, and more Scotsman.

Overall, I loved this series, I can see where hate could be coming from, but I'd give this series a 9.05/10, definitely a great experience, and one of my favorite animated shows to date.

EDIT:I feel Genndy's last shot was breathtaking, and a much better ending just came to me, even though I thought the ending was pretty good, but here's the ending I would've preferred given a bit more time.

Aku is killed (in a two-part-er, thus making the 1v1 of Jack vs Aku longer, Ashi would be fighting, more mentally since she could kill them easily with her powers of Aku, her mother or the rest of the cult), Ashi, since she was born to her mother, can actually only have some of Aku’s powers (shape-shifting.) Ashi nearly dies in Aku’s lair, after Aku is sliced into a single piece which Jack glares down at. Aku, realizing that he is f*cked, decides to purposefully remove his existence so Ashi will die. The Scotsman quickly learns of this and comes to trade his essence with Ashi's; therefore, sacrificing himself in order for Jack to live a peaceful, normal life with Ashi. Ashi lives and all seems well at first, BUT Jack is still tormented, in the back of his mind, by not being able to get back to the past and save all of the lives that were destroyed because of Aku and by his aging conundrum. The ending scene is in the destroyed forest where only one tree remained (in the future of course) and Jack sits there solemnly, when Ashi walks up besides him saying, that hope lives and there must be a way to get him back to the past. Jack corrects her saying in order for US to get back to the past, and just before they kiss, a ladybug lands on Jack and Ashi’s hand; reinforcing that hope lives, they kiss and the forest that Aku devastated finally starts to heal with the presence of pink flowers growing back slowly. *Original outro plays*