I liked it.
I can somewhat understand where people trying to argue over time-travel mechanics are coming from, but that feels silly to me in a series where "Celtic magic!" is a thing.
Instead of thinking about the timey-wimey-wibbly-wobbly stuff, think of it in terms of story and what the last 5 minutes communicate. You can say they did it to just give Jack a shallow, bittersweet ending (and you honestly might be right), but here's my interpretation: The ending isn't necessarily about only Ashi, but the consequences of what Jack did and what it means for everyone he fought so hard to save in Aku's future. Without that world, everyone who just contributed to the final battle might not even exist.
Throughout the series, Jack always made a point to save everyone he came across even if it put his own mission to destroy Aku in jeopardy. There were several times where he could've taken a time portal and just prevented the whole thing, but knowing he abandoned people to some horrible fate (Jack knows about as much about Gendy's time travel mechanics as we do) would always weigh on his conscience. A persistent alternate timeline is still a possibility, if not to Jack then at least to us, so saving them whenever possible always makes sense. And of course, these characters endear themselves to us so we always want Jack to save them regardless.
Before this season began, I wondered whether Gendy would actually have Jack return to the past, and whether that's even the best idea thematically. If he actually returned, that'd be like saying that everything can be fixed by going back to an idealized "Golden Time" (hint hint nudge nudge) and yeah, that fits fairytale conventions but let's be honest here... If it weren't for Jack's conscience, would any of us actually care about the people from that time? I mean sure, we saw them train Jack in the opening episodes (if I remember correctly) but we've spent faaaaaar more time in Aku's World getting to know the people there. Yeah there are a lot of horrible people, robot assassins, etc., but we've also seen that good people can still survive and are worth fighting for in that future... If Jack just went back to the past without killing Aku, we could always think there's an alternate timeline where Aku still reigns and those people are stuck suffering without Jack to help them. That's why a lot of us clung to the idea of Jack training himself to defeat Aku in the future before going back, as The Guardian's vision seemed to imply.
But Ashi gives both Jack and the audience a definitive answer, and it throws that (honestly more optimistic) possibility out the window. The last few scenes aren't about Ashi disappearing to give Jack a sad ending, they're about an entire timeline ceasing to exist. And just like we have to deal with knowing that The Scotsman, Ashi, and everyone else has been erased, Jack has to deal with the weight of knowing all the people he struggled to save in the future are now gone. It's a complete reversal of his position before - where he was once struggling to deal with his failure to save his family in the past, now he has to deal with the ramifications of erasing everyone in Aku's Future, without an easy out like simply disappearing due to his "present self" not existing as some have suggested. Jack still exists as he is because he's the audience surrogate - yeah it might not make the most sense mechanically, but thematically it hits home a bit harder.
Granted all of this might just be me seeing meaning in a shallow, forced ending, but I like to think Gendy's a little more thoughtful than that.
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