The Borg were intended to have a recurring role beyond "Q Who?" in the first place, so again it falls to the writers for not defining them properly.
It WAS a bad idea to make the Borg the number one threat in the Galaxy, mainly because they're so boring once you get past the "Hive Mind" thing. They're only good for 1-2 stories (and history backs me up), and as such the best strategy when having created such a dead-end foe is to introduce other threats of their scale only NOT make them so boring.
8472 were a step in the right direction, they just needed to be introduced earlier in TNG.