That would completely change the story structure. Rian took the 3 main protagonists, identified their wants and needs. And then gave them characters that enable or support thier wants, and a character that forces them to see and accept their need. Rose is the character that forces Finn to see his need, DJ the one who supports his want. And Canto Bight is Finn's schoolroom.
Except that smacks of not writing a story, but structuring activity for 3 different characters and then writing a story to satisfy that requirement. It's story first and foremost.
it's almost like the Indy joke in BBT - take Indy out and nothing really changes.
Take out rose, and for that matter Canto Bight, and what really changes if there was a code breaker on the ship and they just went right over the same way?
Basically, nothing. The film gets shorter by about 15 minutes. That's it. Finn still goes to the ship, gets captured, faces off with Phasma, wins, returns, etc. Structurally, her presence changes nothing. Now...maybe her payoff is in 9, but as far as 8 goes, largely irrelevant. As you say, the only thing she accomplishes is having a buddy for Finn to hang with in canto bight. That's it.
Not sure I get the point of CB to be honest. They go there to find a special hacker, don't find that guy (least I don't think he was), get 'a' hacker (in theory), who sells them out. If the whole point is to get them captured, just send them over on their own and let them get captured. Add a little ANH roaming the death start aspect and sneaking around. That'd be more interesting and builds the characters and gives them buddy buddy time in the process. At least that makes sense. The odds that they know where in the biggest ship in the galaxy the tiny tracker is and exactly how to get there and can get there quickly are about zero. But, again, sneaking over and finding it servers a purpose. The whole CB process didn't/doesn't seem to serve a purpose really.
