They are junk speeders and falling apart. The weapon is about to blast a whole in the very thick door and Finn is trying to fly down the barrel. His ship starts disintegrating around him and his weapons are destroyed. It took no brain power for me to understand Finn was going to kill himself and do no damage the first time I saw it. Nothing needs to be added or explained. None of it is fan theory. The movie properly shows us what we need instead of giving us a boring speech to lay it out. Show don’t tell is basic writing 101.
I disagree. It’s not made clear at all that his sacrifice would have had no effect. But I don’t think it matters. Either way, Rose’s point remains the same.