Conventional Earth forces should fall if confronted by a single Cube.
It's just that the stoopid humans keep mounting unconventional defences.
Even if they send ten Cubes, each new unconventional defence the Federation pulls out of it's ass at the 11th hour is still going to work even against 20 cubes because the Borg will not have a countercounter.
Each new counter works once, and is thereafter considered conventional.
Ergo the Federation will fall to one Cube in the next bout.
And if the Federation will still keep inventing new counters to Borg superiority, isn't it best that they only lose one cube while figuring out a countercounter to any such new counter than losing ten Cubes if total loss is a certainty as long as the ongoing unpredictability of Federation tactics is a certainty?