That’s the point for Poe, that he can now see when a suicide mission is a suicide mission instead of just an opportunity for heroism. The point for Finn is he doesn’t care if it’s a suicide mission or not, as long as he saves the Resistance. The point for Finn isn’t affected by Rose saving him either, because the intention is what matters here.
But my interpretation of the scene is that, had Rose not saved Finn, he would’ve killed himself and succeeded in destroying the weapon and therefore saving everyone/buying them more than enough time. If this was the case, then Rose was very selfish by saving him.
The situation in the end works for every character except for Rose.
Rose is just following through on Poe’s assessment. You could just as easily give that moment to Poe, their intentions are the exact same (and it’s not a selfish act, Rose didn’t “stop him,” she “saved him, dummy”). The reason it’s given to Rose is because that’s her role in the film, the angel on Finn’s shoulder guiding him toward the good. She taught him to become a member of the Resistance, which he follows through on, but he still has more to learn. This is just her next lesson.
Again, it’s obvious the filmmakers’ wanted you to be on Rose’s side here. It’s not something that I thought was an issue when I saw the film, but if you misinterpreted that then I guess they needed to make the pointlessness of Finn’s potential sacrifice clearer.