There’s already a ton of lines, plus the music, plus the editing, plus the visual information, that show the audience that his run isn’t going to work.
The movie is also asking you to remember what a “successful” suicide run would look like - i.e. the beginning of the movie, where Rose’s sister died helping destroy the dreadnought, but that destruction didn’t actually help the resistance, and in fact hurt it immensely. A lot of the same visual and audio cues are there in Finn’s run, and the movie is hoping you’ll remember that, and remember that it ended up being a bad idea, poorly executed, that wasn’t worth the cost.
So you’ve got:
Poe saying they need to get to the cannon
Poe saying it won’t work once the cannon is charged
The AT-AT’s killing everything on the way to the cannon.
The cannon visibly being charged
Finn angrily waving everyone off for the sake of not letting them win (despite the fact they’re going to win even if he does blow up the cannon)
Poe saying the cannon is charged and it’s not going to work
Rose saying it’s not going to work
The ship, which was introduced as being a rattletrap hunk of trash, literally melting and falling apart the closer it gets
The rapid cutting showing how quickly it’s melting the closer it gets.
The music rising and getting more desperate/despairing the closer he gets.
here’s another question - since most of the criticism is coming from an “In the moment” POV, what do you think happens AFTER this scene, where Finn kills himself flying his speeder into the cannon - or the ship fails out and just smashes into the front of it, or goes in and explodes, and now the front of the cannon is blackened/charred but still spinning, or the cannon actually explodes and takes out a couple AT-AT’s on either side but there are still a bunch of walkers and Kylo’s ship (a larger version of which we just saw with the Holdo maneuver) - and the ability to call for reinforcements to just bring another one of those things down and continue the siege: What’s the story after this point? What is happening after the momentary “Yes, Finn is the suicidal hero like Paige was” that is adding to the overarching themes of the story? Why, after seeing two not particularly successful suicide missions complete themselves already in this movie, would the third time be the charm especially considering the tons of evidence being spoken/shown/heard that it 100% wouldn’t work and would be easily the least successful suicide mission of the film, and how would it’s hypothetical “working” make this movie’s aims and thematic strengths stronger than if Finn is stopped?
That scene isn’t happening in a vaccuum, and I don’t know how much more clear the movie needs to be about communicating the futility of Finn’s intent. The scene only works because of it’s obviously tragic nature: He’s going to kill himself, it’s not going to matter, and it’s a damn shame. You empathize with him, you understand why he’s doing it, but you know it’s doomed to fail. Nothing in the sequence there is suggesting his potential success (which isn’t even really a potential outcome at all) will benefit anyone.