As someone who really does like this film, this confuses me too. What was their plan, initially? As has been said, they never fire guns at the thing, assuming they even have guns, so was it always a suicide plan? How else could they have possibly taken it down? But if so, what changes in Poe between launching that mission and abandoning that mission? All of his character beats happen before the mission even starts. The lesson of the day isn’t that sacrificing yourself is bad. Because, you know, he learned his lesson from Holdo, who sacrificed herself. The idea, as I understand it, is supposed to be to understand that sometimes discretion is the better part of valor. That it’s foolish to do pointlessly heroic things for pointlessly heroic deaths. That’s the contrast between Poe’s Dreadnaught attack and Holdo’s sacrifice. Poe causes a huge part of their fighters to get killed for a glorious victory when they could have simply turned tail and lived to fight another day. Holdo killed herself solely so that her comrades in arms could live to fight another day.
However, what Finn was about to do feels much more in line with the latter example than the former example. Finn was going to sacrifice himself, not for glory, but so that his friends might live. This is exactly what Holdo did. So I just don’t get why Rose and particularly Poe are against it, especially since I don’t understand what Poe’s plan was in the first place.
Their initial plan was to destroy the ram. It’s not stated explicitly, but the skimmers do have guns, so the assumption is they’ll use those. Yes, we don’t see them fire the guns, but that’s because they need to get close enough first. However, once they get closer, the ram starts charging up, which (as we see on Finn’s skimmers) destroys the guns. Thus, the only way to stop the ram would be to use the skimmer itself as a weapon. It is decided that is not an acceptable plan.
The situation is much more in line with the Dreadnaught assault. The Dreadnaught is about to fire on the main cruiser, but instead of running away, Poe has his fighters sacrifice themselves to destroy it. Trade out “Dreadnaught” with “battering ram,” and “main cruiser” for “shield door.” In the case of Holdo, the situation is completely different. The Resistance transports are already running away, but not unnoticed by the First Order (which was their plan). Holdo was only supposed to be leading the FO away (and the one loss when the cruiser was eventually destroyed, instead of many). When she decided to crash into the FO fleet, it was because she had no choice. If anything, the link between the Holdo scene and the Finn scene is that Finn was trying to copy her sacrifice when it wasn’t applicable.