And no, the villain decay isn't 'inevitable'. I originally said that, but I thought about it. I thought about the Dominion. And throughout DS9's run, the neither the Jem'Hadar nor the founders lost the menace they had when they were introduced.
That's because the Dominion were allowed to win battles and stuff. They could destroy hundreds of Fed ships and kill lots of people without harming the main characters, which allowed them to keep their menace. With the Borg they couldn't be allowed any victories because all VOY had was their one ship, and they couldn't let them destroy THAT.
So basically, VOY was again in a no-win scenario: Their dramatic situation meant their enemies couldn't be allowed victories that would endanger the ship and crew otherwise the show would be over. THus, their villains could never keep up a massive menacing aura.
And yes, you DO need a constant enemy (even in a show like VOY), NuBSG and Farscape had them as well.
And no, the writing wasn't the problem. The writing for guys like the Vidiians was no worse than anything on DS9.
...Which would make this...(gasp)...A double standard!