I actually really like that all four pc's in SP can talk to people. One of my favorite moments in Fort Joy is with Butter. If you hit on her with one character, and then claim that you aren't really interested in her, and then if you try to hit on her with a different character she will outright refuse that characters' advances because she has already heard that line. Also in a lot of instances if you say certain things as one character it locks you out of talking to that character with others.
I think more instances of the game remembering which characters have already gone through which dialogue, and changing the options these other pc's can say, and changing what responses npc's can say, would make for a better experience then what you are suggesting.
There are only very instances in which this is the case. Most NPCs just act like the previous dialogue never happened and that's imo pretty bad. I really don't want to go through the same dialogue ever and ever again.
I can only imagine how much work and time would be needed to enhance every possible dialogue in the game in order to give it the proper reactivity to four possible dialogue partners, remembering all the stuff that has already been said. I doubt that Larian is able to do that so I would rather opt for one of the solutions I suggested above. I mean, I rather have a stringent but clear design approach than a system that offers pure repetition in 90% of the cases.
And even if you put all that effort in making all the NPCs remembering all the dialogue lines with different characters there are still issues to be thought about, like replayability and player behaviour. In a system that offers possible specific dialogue for all four party characters the player is motivated to talk to every NPC with every single character of the party. I think that this is tiresome in the long run, especially in SP. I'd really prefer a solution that would require me to play the game several times with different characters than to be subliminally "forced" to use all my characters for the very same dialogue partner all the time.
You see, I like your example above, I really do. But it's just a one shot example that sounds promising on paper. If you have to go through every dialogue up to four times in order to find such instances it gets incredibly tiresome very soon. It's just busy-work, much like looking for crafting-materials an such in other RPGs. And I really don't think that this was the intention of the writing team, to drag out the narrative and dialogues in this specific way.
