Except thats kinda what ESB does..... In both films, they take our heroes, which we left celebrating a hard-won victory, and thow that in the blender. And put then they put those characters through the wringer. The story is written to facilitate this. For instance, the tunnel conveniently collapses(Seriously the Star Wars show needs to do an "Everything is Important" segment on that tunnel, or would that be General Veers....), meaning Han and Leia are now forced together, and the hyperdrive is busted, so their time spent together is even longer. Allowing a deep relationship to form. Now each film does that differently because you know, they have different writers an directors.
That's also how George writes. He's said he comes up with several things he wants to see happen in the film, and then figures out a way to connect them together.
So what would change if Rose was taken out? Finn would never become a Rebel. He isn't fighting for a cause, other than his own. Which is to save Rey. I mean he says it like 5 times or something. Actually his relationship with Rey is a little similar to Anakin and Padme, in that he has kinda this selfish love for Rey(not necessarily a romantic love). The kind of "I can't live without her, I'm going to do whatever I can to save her, even if that hurts other people" kind of love. That's why Canto Bight and DJ are so important because Finn sees where that selfish greedy attitude gets people. The kind of monsters they become. And why that scene showing him being willing to sacrifice himself for the Resistance, is important. Earlier in the film, he wouldn't have done that, unless Rey's life had been threatened. But know he identifies with the cause. He's a "rebel scum." He's not Finn who wants to run away from the First Order with Rey, anymore.
Anyways that's how I see things. Me personally, I'm thrilled to see more Rose Tico.
