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.
Uh this cannon! Reminds me of this:
Not a very good idea for Star Wars.
GROND! GROND! GROND!
That’s very much what they were going for, in fact I am pretty sure the cannon was referred to as a “Ram cannon” at one point. Star Wars has always had that fairy tale, Knights of the Round Table, ingredient - so it seems quite fitting that they should ‘suggest’ a medieval castle siege.
The Resistance Base is the castle, AT-ATs are the ballistas, the Ram cannon is the battering ram, and the speeders are the brave but foolhardy cavalry charge.