I think it's the knowledge that no help is coming that determines that Finn's attack would ultimately have been a pointless gesture (disregarding the question of whether his plan could even have worked, given his ship was falling apart and it was unclear if it'd reached the cannon).
If help was on the way, then destroying/disabling the cannon may have bought the rebels enough time for that help to arrive. Once we learn nobody is coming though, it hardly matters. Destroying the cannon would just result in a long, drawn out siege where the First Order starve the rebels out - and that's assuming they don't have spare bunker busters to deploy.
As an aside, Rose's plan to 'rescue' Finn is equally dubious. If help was on the way, she could just have cost the rebels the one chance they had to buy some time. And crashing her craft into his could (and almost did) result in two pointless rebel deaths instead of one (maybe more if others had gone to rescue them). Besides which, the entire franchise is built on the heroic sacrifice trope, right from Obi-Wan in ANH through to Luke in TLJ just moments after the movie tried to make some point about such sacrifices.