Finn was defintely on a suicide mission and he might have damaged the weapon, but not likely. Remember in Independence Day he has weapons to fire. Finn has nothing. He is trying to run into a Locomotive with a Citroen. It isn’t going to do more than scratch it. The angles they show to make it clear how small the speeder is, the parts crumpling from the power, Poe’s earlier comment about how flimsy and decrepit the speeders are (when he puts his foot through the side). It is all story telling to indicate that Finn’s run is hopeless so we are relieved when Rose stops him.
“Never underestimate a Citroen”
I still disagree that the movie is clearly telling us that his suicide run would not disable the cannon. For me it’s not made abundantly clear either way and maybe that’s intentional. As you said about Poe’s decision at the beginning of the film, whether or not Finn damages the cannon is immaterial to the point that the movie is trying to make, so it seems entirely appropriate for it to be kind of ambiguous.
Exactly. If it was clear that he wasn’t going to be able to destroy it Poe should have said something like ‘It can’t be destroyed now!’ or ‘Don’t throw your life away for nothing!’ But instead he specifically says that it is a suicide run, just like Rose’s sister and the bomber at the beginning. The whole point seems to be that whether or not the weapon is destroyed it isn’t worth the lives spent in taking it down. The problem of course is that in both the opening battle and in this one, taking down the weapon will save the Resistance based on the facts at the time, and not doing so dooms them.
But this discussion has been had and nobody is changing their minds now.