It is my understanding that for some time, the Lich King had built up forces significant enough to turn all of Azeroth, Horde and Alliance alike, into a charnel pit. The vague sliver of humanity left in Arthas- now virtually outside him, like a ghost, has successfully managed to prevent unleashing this war-machine fully upon the world, but it is nevertheless ready and aimed at us- like a taut bow, drawn and ready to fire- kill the archer, and the arrow flies (messy, but it still goes someplace).

I'm positive that despite Uther's insistence that, "There must always be a Lich King," it is absolutely possible that werf could don the crown and proceed to dismantle the Scourge forever- if werf could retain werf's senses, personality, and intentions well enough to do so. If we physically dismember and disintegrate ever last undead, seal away or decimate every plague, etc.- then there would be no Scourge to fear and no need for a Lich King.

As it is, however, our entire efforts have focused on cutting off the head of our foe- that is, killing the Lich King. I think the main, final point of all this is simply this: It won't work that way. Just like a zombie, cutting off the head is pretty much a useless gesture- in this case, it may actually make the body (The Scourge) MORE dangerous, rather than less. Uther and Blizz are saying that we essentially have to adapt our goal to current information- that is, we're going to cut off the current head and through somebody else's head on it, one that we won't have to fight, at least, not as hard as we do Arthas/Ner'zhul.

I'd like to add that our plan to cut off the head is a good one, despite everything. Go into Pit of Saron and imagine each of the slave and NPCs in there as at least equivalent to yourself in power, even though, for mechanics reasons, they aren't. The Scourge is FREAKISHLY SCARY and practically invincible- you can't just fight it, because for every bit of life you lose, it gains. You can't beat it down through a war of attrition. You can't slowly wear it down. Almost every enemy we face from the Scourge is there because at ones point, somebody good and noble and tragic fought them and lost. By all kinds of means, it is constantly growing, like cancer. It's scarier than Chuck Norris, is all I'm saying, and we felt that 'kill the Lich King himself!" was probably a really good way to cut out the tumor that we call the Scourge.

And then Uther effectively pops up and tells us, "No, no, that's not gonna work. I'm sorry. It sucks. You're all just gonna die. Toodles!"

And while we cry our sorry little eyes out, He pops back up and says,
"Oh, don't be such a pansy! Anyway, I thought of something neat- kill the Lich King and then TURN ONE OF YOUR BUDDIES INTO THE NEW ONE. Yeah, I know it'll suck, but, come on, you had a second to get used to unavoidable undead servitude, right? Go find some sucker to pawn this off on!"

What do you think? Added perspective helps out on why the Scourge will go psycho instead of gaining free will like the Forsaken? Or maybe not?

I should add lastly, the Forsaken have indeed NOT given up the Scourge quest of destroying everything- they're just a good bit sneakier than the Scourge. RAS, Sylvanas, Putress, and Wrathgate-related lore as case in point.