It really seemed like Finn was more than able to destroy the cannon to me. He was still going relatively fast in the cannon’s direction, while all the others turned back, which it in itself is a plot hole (how did Rose even get to him?) and something I noticed upon first viewing.
The cannon’s beam appeared to slow down anything in it’s path - and was also stripping away parts of the ski-speeder (including the ski-speeder’s guns).
Dialogue from the Crait battle… (taken from another site - so for whatever it’s worth on accuracy)
Rose (to Finn): Hey! Engage your mono-ski.
Poe: All right, listen up - I don’t like these rust buckets and I don’t like our odds (‘What the hell?!’ - as part of the floor falls off). Just keep it tight and don’t get too close until they roll that cannon out front.
Poe: All right - Ground Forces - lay down some fire!
Hux: Thiteen incoming light craft. Shall we hold until we clear them?
Ren: No… the Resistance is in that mine. Push through.
(Tie Fighters enter the fray)
Poe: Fighters! Break off! We gotta hold them until they pull up that cannon!
(The Falcon enters the battle)
Ren: Blow that piece of junk out of the sky!
Hux: All fighters! (after the Falcon)
Rey: Chewie, peel off from the battle - draw them away from the speeders.
(The Falcon leads away the Tie Fighters)
Poe: She drew them off - all of them.
Finn: They hate that ship!
Rose: There it is. That is a big gun…
Finn: Okay. It’s heavily armoured - our only shot is right down the throat.
Finn: The cannon is opening (emitting it’s beam) - this is our chance.
Kylo: All fire on those speeders.
Hux: Concentrate all fire on the speeders!
Rebel on radio in base to Leia: We’re taking heavy losses.
Poe: They are picking us all off - we are not gonna make it.
Finn: Alright - it’s my final approach. Target in sight. Guns are hot.
Poe: No - pull her off.
Poe: The cannon is charged - it’s a suicide run. All craft pull away.
Finn: No! I’m almost there.
Poe: Retreat Finn! That’s an order.
(rest of craft break off - Finn stays on course)
Rose: (indecipherable) - its too late. Don’t do this.
Finn: No! I won’t let them win.
Rose: No, Finn - listen to Poe. We have to retreat.
The cannon’s beam start to hit the rebel base door.
Finns’ craft’s guns melt in the cannon’s beam.
Finn disengages the stabiliser and flies up level with to the cannon.
As he’s about to enter the cannon mouth Rose hits him from the side.
Finn: Rose! NO! (runs to Roses’ damaged speeder). Rose! Rose? Why would you do that? I was almost there. Why would you stop me?
Rose: I saved you, dummy. That’s how we’re going to win - not fighting what we hate, but saving what we love.
Cannon explodes the rebel base door.
Rose kisses Finn. Passes out shortly after.