Plausible stops being a thing when you’re watching a movie that already has magical powers and lazer swords.
I don’t agree with all of Lizard’s nitpicks, but this specific statement is not true. Every film has an internal universe, where it establishes its own rules. In Star Wars, the half a dozen stormtroopers might fire their blasters in Han’s direction, and none of them are good enough shots to hit. But that’s a recurring movie trope that is consistent throughout the internal universe of the story.
However, if one of the stormtroopers in Cloud City had burst into Han and Leia’s room, and warned them that they were in danger and they need to hurry and get away, that would be seriously out of line. That is no longer an insignificant, oft-repeated trope, but that is suddenly a major WTF moment that (at a minimum) needs explanation, and probably shouldn’t be included in the story at all.
That’s why Finn defecting is not consistent with the internal world of the Star Wars universe. It could still work … but now the whole thing calls so much attention to itself, it needs some kind of context to justify itself. Just saying he’s a good guy with a conscience doesn’t cut it.