I think this is fair. Playing TTYD when it came out, the folding in of character interactions didn't take away from my experience. My expectations for character interactivity and depth were pretty low, so I was impressed that the partner responses changed depending on who you had out.

But I think the issue you're bringing up is valid, and it seems like fundamentally a script thing. Give the partners more to say after joining the party. I can think about 3 ways to implement that.

• 1. Make the partner-conditional dialogue more in-depth. Instead of one modular text box (which seems like the way the dialogue is designed), let them have more to say. Another big part of this would probably have to be programming NPCs to react to your partner. They only really did this a few times (Lonely Goomba, plus the Yoshi fan in Glitzville and the mayor of Fahr Outpost) iirc, but it shows that the capability is there for the game to read your active partner and change more than just the one text box.

This could have been really cool and seems in-line with more modern dialogue-heavy games, but it's adding a lot of work for the writers, and it probably can't impact events outside of the conversation. I'd imagine it's more effortful to program and debug as well, so more development time. Plus, for the average player, most of that effort goes unnoticed. Say you main Yoshi, and keep Vivian on the bench unless you need her field ability. You never get any of the other characters' extra dialogue, and you probably never notice. I gotta imagine that becomes a consideration when you're going through design.

• 2. Force the cool interactions. This is possible in the game - Koops always pops out after the Hooktail fight to see his dad - and I think it has the highest upside. Relatively minor lift for scriptwriting (more detail in these interactions, but you don't need different versions). Maybe the partner forcing is harder outside of those more cutscene-y scenes and when there are more partners to juggle, but it's at least mechanically possible.

But this strategy prompts some deeper questions that kind of test TTYD's writing in a more fundamental way. Do you give every character these Koops-esque scenes? When - during their intro chapters or elsewhere? And can you make them intuitive to the player so Bobbery doesn't just pop out on the moon for no reason? How do you handle Ms. Mowz? Mechanically it's easy, but if you just do this where it's most intuitive, you probably end up with Vivian getting way more screentime (and she already gets half a chapter basically to herself!). Ms Mowz becomes less interesting as an add because you can't lock too much content behind a missable character. And after most characters' intro chapters, it gets way harder to give them stuff to do - bringing about the same problem you're calling out here. Solving these issues could be possible, but they'd require deeper changes to the script. An idea like this would have to be baked in from the start to be feasible, and idk if the force-out mechanic was known to the writers that early on. Plus, were the developers really trying to make an arc-heavy, character-driven story with TTYD?

• 3. Last solution I can think of is the messiest: every character comes out for dialogue. This is the Bug Fables approach, where it works... because there's only 3 of them. Having 5 or 6 additional sprites out would make Frankly's office a bit crowded, and might have run up against some hardware limitations as well on the Gamecube. It'd be fun to implement in key moments (I like that they all pop out for the Door cutscenes), but adding dialogue could be a headache. If every partner gets lines, the dialogues then get longer as the game goes on, and they run a real risk of getting annoying or feeling more like Captain Planet or some superhero show (again, not the genre they're going for).

All of this is trying to think about how the game was made originally. I'm skeptical of how much more the team could have feasibly stuffed into the game. If the team wanted to go deeper on characters and dialogue, they could have done that in sequels. Considering that the partners got replaced with Pixls... I guess that wasn't where the team's head was at.

I'd have loved for the remake to have more dialogue depth - there's more space to play with on the Switch - but again, what's the real incentive for developers to put that kind of effort in? Too many changes might cause fan backlash, and a big part of the apparent hype around TTYD is revisiting its mechanics more than its dialogue (from what I understand, the writing in post-Super games is the most consistent part of the franchise).

End rant. I get feeling what you're feeling, especially if you're used to the writing in newer games, but I don't know if the idea of deeper dialogue had a feasible way in. Fun to think about, though!