SPOILERS: The last season of Demon Slayer started out and for the majority of its run, seemingly was going to end up at a perfect 50, and even seemed to have been going higher with Ubuyashiki's speech to Muzan regarding human will being true immortality. It felt organic and Muzan still wanting to kill him made sense despite the clear consequences spelled out for him, showing how simple minded he is. I genuinely really enjoyed that scene, as well as the result of it, in using himself as bait to blow them both up. I was genuinely engaged for the first time in the whole season. Right at the end.

In the entire span of the rest of the season, we learn zero about any character or grow really any of them besides giving the earth Hashira his backstory. And again while it is a sad backstory, I don't care for him any more as a character than I did before. They also give "closure" to Zenitsu's whole deal of being powerful only when he's asleep, because he read a letter. He read a letter that we don't get to know anything about, holding it out for his fight for hype. The "build-up" of Zenitsu only being powerful while asleep has been a reoccurring thing for the character where I was waiting for them to do a proper character growth segment for him, instead we get a plot device that simply gets him there in time for the movies.
That was my second biggest gripe with the season, but the world-breaking shit they pull at the very end to explain the transition into the Infinity Castle Arc completely shatters everything that's been built up surrounding the first look into the Infinity Castle. Before I rip that apart I want to preface that genuinely, this transition from the Training Arc to the Infinity Castle Arc could have actually been an incredible setup and payoff of a real "oh shit" moment. If they had established that only when near Muzan, or something along those lines, the Infinity Castle can be used to transport people into it, where it would have actually been a genius move to have all of the Hashira's gang up on him at once, outsmarting an outsmart move. It would have made a little more sense and had an actual impact in the end being outsmarted anyways rather than still having the same result and it feel like actual contrived bullshit. To the real ending of this season, everyone gets pulled into the Infinity Castle, even people miles away like Inosuke and Murata. It solidifies in the canon, that the Upper 4 demon can literally just summon people at any time, meaning that for the entirety of the existence of the Infinity Castle, they could have pulled in any Hashira, at any time, or really anyone they have eyes on, and immediately kill them in the Infinity Castle. It's a plot hole bigger than the amount of time I've wasted watching all the episode segments that have played twice in this series. The only people free of this ability would be people in hiding like Ubuyashiki and Nezuko at the start of this arc. It makes zero sense why it hasn't been utilized at all this whole time.
So with the best this season had, with one good scene and the rest being a straight 50, brought down by one semi upsetting decision and a series destroying plot hole, this goes pretty low.