The more I look back on it, the more terrible this finale was. Fans had 16 years to see the emotional payoff of the series. Added to that is the fact that most fans were kids when the show was first on, so 16 years makes a much bigger difference to them than it does in the grand scheme of things. So, after 16 years of waiting, the final season was mostly written around a new character, Ashi, which isn't inherently a terrible thing, since the character was interesting enough and supplied extra motivation for Jack. The problem is that they waited until fans were attached to the character to kill them off, which feels like a blueball ending, especially when she wasn't even a character in the 4 other seasons of the show. It felt like a total waste of a character inclusion and dampened the emotional payoff of the series.
Furthermore, the show had implied in the past that Jack would have to defeat the Guardian to go back in time and defeat Aku. It's understandable that the show would abort this arc (there was a decent chance that most fans had forgotten that this altercation ever took place), but the fact that we are shown a broken pair of glasses makes it feel like a deliberate middle finger to the fans' expectations.
The final episode is the real disaster of the series. Everything is wrapped up all too quickly, the "final battle" is one of the most mediocre fight scenes in a show that probably bested even most Sh?nen anime that were on at the time. Even if Jack hadn't gotten sack-tapped for all of his efforts, it would've been a "Return of the Jedi" or "Avengers: Endgame" ending, wherein the writers just quickly slapped a band-aid on the series' main crisis and called it a day. So really, all the garbage in the finale was just the icing on the cake.
What they should've done is made a finale movie. It would've neatly bookended the series with another three-part epic, as well as averted the writers' temptations to add all of the shit in the finale.