It's not about whether Voyager can destroy a cube or not.
Chipping away at the external hull was useless because there wouldn't have been enough time to do any decisive damage. It was a battle of attrition. Starfleet's entire force was being picked off, one by one. Yes - Hopeless.
And we must accept that Voyager, a single starship, can dance around like a fly and survive attacks from these very same cubes. Or that Seven's parents in their little boat can walk right up to Borg Central and spy on them...
Now we can dream up all kinds of explanations for these things until the cows come home, but at the end of the day it doesn't negate the statement: "Voyager weakened the Borg." It just updates it to: "Voyager weakened the Borg, and then I used my imagination to strengthen them again."
If Voyager was built after BOBW and after the Defiant.
Wouldn't Starfleet also keep in mind that if the Borg are coming that all new ship build after the Defiant would have to be equipped with defenses to fend off an attack? Why would they keep making ships that weren't safe and keep sending good Officers out in them? It's why they kept stressing how top of the line Voyager was and why it had gel packs to make computer processing as fast as thought..............similar to a Borg Cube.
Without the Tri-Cobalt torpedo's, Voyager can't beat a Cube but it's designed to take a beating, defend itself and run.