The Deca-Dence twist was great, so why had I stopped caring by the end?
There is a simple answer to this question. It turned a movie-type plot into an anime series. In a way I applaud the Scamp summarizing the problem into a sentence, but it doesn’t quite unpack the full issue. Deca-Dence spoilers ahoy.
I too thought Dece-dence was a fun show that is quite compelling maybe up to the point where the plan was put together to blow up the monster farm. The narrative has already taken a split between POV of Heybot Kaburagi and POV of Natsume. In as much as the humans in the show are just there, or maybe better put, victims, Natsume gets relegated to that role which gave the beating heart of the story a bit too much emotional distance.
This means Kaburagi’s side of the story has to tag team, and frankly this Heybot-invoking story just doesn’t have the intrigue to carry the intensity the same way that Natsume-could-literally-just-die-any-time, while surf-spearing alien-looking bugs. In a way, the Heybot gimmick worked against it when you have this contrast and it isn’t consistently played for interest, as the second half of the story was mostly in Heybot-POV.
Which is to say, the final boss critter was a powerful monster but was way less fearsome than the doomed-to-fail-but-not mid-series mission where the game devs planned to kill off the over-accomplishing players and humans. Deca-dence was never a game in the first place, by playing up the game aspect, the story has emotionally cheapened its core asset.
Which is also another way to say, the story while had enough gas to go all the way, it was probably too introspective too quickly. The gimmick had to survive 12 episodes (roughly 240-250 minutes if you take out repeated parts), which is probably 120 minutes too long all said and done.