We have no idea how many of these mindless scourge are still alive after the Fall of the Lich King while the majority is probably in Northrend and Bolvar can currently only control those in Northrend.
And yes, they do have a hunger for living flesh and would consume anything in their range but how exactly would mindless know where to look for? How would they get to EK/Kalimdor/other?
Yes they can still be dangerous if mindcontrolled by whatever malevolent being giving them a direction. However the AC/DKs could still decimate their number over the time given. I mean they had about 5x the time of the Northrend campaign.
We were killing high ranked demons and leaders ever since the WotA. Didn’t stop the lesser demons from invading over and over again. Yes, they were probably under some pressure, because if they didn’t invade they would’ve been punished by the management, but that logic is flawed in WoW.
We killing off stronger enemies however never led to lesser ones surrendering unless they were actual slaves against their will.
Besides that: the only cosmic threat left would be the void lords, which aren’t even existing in our plane yet.