To be honest, the first thing I thought of when I read this ask was the C3 defenders who have been insisting that so much criticism of the finale comes from people who dropped off a while ago and therefore wouldn't know. But what you're saying here points to the actual problem behind why the final episodes of the campaign were such a mess. If it's episode 80 of 121 and there's only a perfunctory sense of motivation from the characters? That's a problem, and it's going to make it nearly impossible for the finale to stick the landing. To quote @wardensantoineandevka, it's an Act 2 problem, not an Act 3 problem.
I've heard more than once that "nuh-uh, Bell's Hells does have motivation, it's called altruism", and I'm going to take a detour to explain why that's not enough. If you've followed me long enough to know the deep lore, you know I used to be a fan of Voltron: Legendary Defender, whose final seasons were notoriously disastrous. Many fans hated it (for different reasons), while general audiences mainly thought it was mid. Deeper research into the production history nets inconsistent results; a lot of unsourced rumors and "common knowledge" got spread around Twitter and Tumblr about why the show fell off the rails so hard, and it's difficult to parse what feels true from what actually happened.
What I do know, however, is what I actually saw in the show, where the main cast feels as if their motivation to be there is "we're the protagonists". There's very little development of emotional connection between the characters beyond a surface level, and the characters don't have a personal investment in what they're doing. (And no, "they're just altruists" is not sufficient motivation. The altruism, like the characters, is pretend.) They're there because they got to the giant robots first. So at the end of the show, where they've escalated the stakes to "the whole MULTIVERSE is going to be destroyed", it lacks weight because none of the work has been done throughout the show to make it feel like that matters to the characters. Act 1 was promising if a bit shaky, but Act 2 is a mess, and it turned Act 3 into sludge.
To bring it back to better stories: why is it Vox Machina dealing with the Chroma Conclave? Because it's the right thing to and because their city and home that they had invested themselves in got suddenly and violently attacked, and by a group associated with a dragon they previously killed, and they picked up more reasons along the way. Why is it the Mighty Nein dealing with Cognouza? Because it's the right thing to do and because the major player involved was piloting the body of their friend who died in an incredibly traumatic and game-changing moment, and they picked up more reasons along the way.
Why is it the Hells dealing with Ruidus? Because it's the right thing to do and...because Imogen had moon dreams and Orym's family was killed and everyone else is sort of there. Why is it Team Voltron dealing with the multiverse problem? Because it's the right thing to do and...because they're the ones with the giant robot. More than one person has described the vibe as "we met during freshman orientation and talking to anyone else would be scary".
The Hells are not played with the level of intentionality that this plot requires—but ultimately, as many people have pointed out, most of the burden of this falls not on the players but on Matt. Being so hands-off during character creation meant that he allowed the cast to make characters better suited to a completely different story than what he wanted, and was either unable or unwilling to pivot to accommodate. ("Pulpier and deadlier" is getting passed around and dunked on for a reason.) The cast was mainly trying to thread the needle of playing true to their characters while also trying to meet the needs of Matt's story when he was frequently keeping them in the dark about what he wanted for the sake of surprising them.
When the big setpiece moment of episode 51 came and went, the campaign became so focused on getting everybody through plot points that the only conversations they had were the seemingly endless circular god debates that went nowhere. It's not really a "nuanced morally gray story" as its defenders claim; it's the DM seeing the party go in a direction and then throwing something else at them to "complicate" things in a way that either gets forgotten about entirely (Hearthdell) or just grinds the story to a screeching halt for no payoff (Feywild trust exercises). These problems are most noticeable in the final arcs (particularly with the Arch Heart appearance—not giving Abu any direction and just letting him improv was a very poor decision), but the feeling that Bell's Hells are just a ping-pong ball bouncing from fetch quest to vaguely-related fetch quest, rather than active agents in their own story, was present well before that.
Campaign 3 probably won't be remembered as bad. At the end of the day, I think it's just mid. And honestly? That might be worse.
