The weapon is about to cut through a door several feet thick so Finn’s craft is likely to just vaporize if he flys down the barrel. No weapons and no ship to make impact (and a flimsy one to start with that wouldn’t have taken out an AT-AT). So Finn was making a useless sacrifice because he was desperate to do something.
Yeah, but that’s all fan theory and retroactive justification. The movie didn’t make that clear enough, which accidentally gave people the impression that Rose doomed the Resistance, which wasn’t Rian’s intent with that scene. That’s my main problem with TLJ. Rian had a good vision for the movie, but a lot of his ideas weren’t executed well.