His subplot had a more direct and tangible effect on the actual plot of the film than Rey's.
How does the First Order find out about the Resistance's plan, which was 100% working as intended, without Finn, Rose, and DJ being captured, and DJ, the guy Finn and Rose trusted, spilling the beans, after overhearing Finn being told Holdo's plan? Which eventually led to the hyperspeed ramming and then the Battle of Crait. You'd have to do major rewrites of the film if you were to remove Finn's plot. Meanwhile, how does Finn contribute in meaningful way in the Rise of Skywalker? THAT'S an example of a character being easily removed and it changing little to nothing.
Finn was following Poe's lead. Poe is the defacto leader by the end of the film and thus all of the fuckups can be attributed to him. Finn's sacrifice would've been as effective as Holdo's. I.e., the FO wasn't gonna be stopped. The film very blatantly telegraphs that this is all Finn's "sacrifice" would've amounted to:
Finn's redemption is based n him becoming fully committed in the first place.