Edit 2: This is sort of an open ended title I guess, so maybe we can come up with other seemingly obvious things they should have done.
In that case, why didn't the Borg send 150+ cubes into Federation space, one for each member world? Starfleet would never have the numbers possible to defend against each one.
Yes the Borg could have quite easily destroyed the Federation if just one Borg cube kept most of the Federation finest ships at bay.
If we hadn't had the insight into the Borg world in VOY you could just explain it as being one of the mysteries of the Borg. Now it just doesn't make sense.