1) Most of the Scourge is made up of ghouls, abominations, and other nearly mindless creatures. They don't really have any free will to get back. Even if they didn't run rampant, they'd probably fall under the sway of various Cult of Damned necromancers and still be a problem.

2) The Forsaken are kind of nuts because of their time spent in the Lich King's service. Now imagine how nutty someone would be who had been serving the Lich King 5-10 times as long.

3) Speaking of the Forsaken, look how well that whole "undead with free will" experiment turned out. Not even the Horde really trusts them anymore, so it makes sense that the Alliance especially wouldn't be thrilled to see either the Forsaken's population grow or an entirely new faction of free Scourge.

4) The Lich King only recently became a major threat. For years he sat around doing nothing. By putting a new Lich King on the throne, the hope is that he'll be able to hold back the Scourge for a while. Considering how messed up Arthas was to begin with, I'd expect the new Lich King to be able to stay relatively uncorrupted and harmless for much longer. I'm sure the factions will be hard at work trying to figure out a way to destroy the Lich King once and for all, but until then this seems like a pretty good option.