I suspect that even the Borg, as they cruised the spaceways in their unassailable Cube armada, minds linked into a single collective superconsciousness, would occasionally look at each other and go, “Man, what is our deal? What the heck are we supposed to be doing, anyway?”
Obviously their quest to assimilate other species was born of a painful awareness of their own vast mediocrity. They were desperate to acquire that elusive quality that would transform them into really first-class adversaries. You’ll notice that they never assimilated short races, for example.
Seriously, while the rock-opera space zombie schtick made them an interesting change of pace from the usual shoulder-bepadded Trek adversaries, they should never have been allowed to become a recurring foe. I mean, they had no dialogue! How can you have an enemy with no freakin’ dialogue on a character-driven show? That’s like having multiple episodes where the Mugatu attacks.
Ideally, the Borg should have been held in reserve at the edge of the Galaxy, a remote but ever-present danger, until finally allowed to swoop in and deal out some serious widescreen hurt on the Federation and its neighbors. I wanted to see whole planets assimilated, colonies scooped from planetary crust and hurled into space, humans, Klingons, Romulans, everybody scrambling to defend against an overwhelmingly superior enemy. Instead we got endless sorties and meditations on the nature of individuality. Hey, screenwriters; maybe you hadn’t noticed, but you covered that territory in the first Borg story! Yes, the Collective is bad for individuality! We get it! There’s nothing more to say! You spent that nickel! Move along, please!
Oh, my god… I’m getting a psychic flash… something…something about the upcoming episode of Enterprise where the Borg are slated to appear! I’m getting the sense that…yes, they will try to assimilate someone… the Enterprise crew will intervene… and the Borg will be defeated! And the moral is that assimilation is bad, and individuality is good! And in the end it doesn’t really matter!
In other words, the Borg didn’t jump the shark; they were the shark.