NeverarGreat is bringing up some great points. I think the very fact that so many people (at least that I have talked to) thought that Rose crashing into Finn made no sense proves this. Because they felt (as well as I) that the movie set them up for Finn making a successful sacrifice.
That’s just how they interpreted the scene, as well as how I did. Doesn’t make it more right than an alternate interpretation, but that is kind of how movies work. They are meant to provoke different opinions from people and readings of the same material.
The Last Jedi in my opinion, for that scene, presented it in a manner that set us up to be angry and puzzled with Rose’s decision. Just my reading of the scene.
I think most disagreements over TLJ originate from people presenting their interpretations of certain scenes as the only valid—and therefore obvious—interpretations, with any interpretations running counter to theirs being completely wrongheaded.
I’m not trying to say every scene has only one proper interpretation, but rather that your preconceptions of how the story should go color how you interpret a scene. Knowing this is Star Wars and the heroes don’t die, Finn’s life is never at risk and there is not chance of him destroying the weapon because he has to live for the next film. That is a Star Wars Trope. So watching that scene and expecting to see Finn die is not a reasonable expectation of the scene. The purpose of that scene was to show that during the course of this film, Finn went from trying to save Rey to trying to save the Resistance. He has gone from a runaway Stormtrooper to a full on Rebel. That was his character arc. Rose helped him on the arc and then rescued him when the arc was complete. The details in the movie back this up. A speeder needs speed to do damage. One of the things to notice is that his speeder is moving slower in the growing force of the beam. Finn is on a pointless suicide mission because he wants to save the day. When you look at all the things in the movie that indicate it is pointless, Rose saving him becomes very natural. It isn’t the only interpretation, but it is the one that fits what we see, his story arc, and what is normal for Star Wars.
Others have already explained why the opposite interpretation is also valid, so I won’t rehash that. If it were as clear as you suggest, viewers would be more uniform in their interpretation.
As far as Star Wars tropes and “what is normal for Star Wars”, the director doesn’t get to subvert expectations in one scene and then fall back on tropes in the next. In New Star Wars, vessels can hyperjump into other vessels to destroy them and characters can develop Force sensitivity into Yoda powers within weeks. Why should I expect anything I’ve seen before to hold up in this new universe?
These two complaints are ridiculous. First, a vessel hyperjumping into another vessel (or planet) was established in ANH. “That will end your trip real quick”. Second, Rey’s force powers were developed in TFA. Every single one she picked up after “seeing” Kylo Ren do it. She really didn’t pick up anything new in TLJ. So neither complaint is accurate for TLJ.