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.
If the beam is strong enough to crumple his guns, it’s probably strong enough to slow him down. That would increase the doubt that he was actually going to do any damage and increase the likelihood that Rose could catch him and stop him. But aside from that Finn also is not an expert pilot he had to be told how to operate this thing and he probably isn’t flying as fast as the little skied thing can go.