You may also want to keep in mind that the Sierra IP may include newer games too. It appears that some games published by Sierra after being purchased by CUC are still part of Sierra while others have been transferred to Activision. However, purchasing the IP of all the old Sierra franchises may be negotiable, if Activision refuses to only sell the King's Quest IP alone.
Of course, a commercial license would likely be cheaper, even if the KQ IP could be purchased by itself, since Activision could negotiate a slice of the profits or give themselves some other advantage through the license rather than give up all rights to KQ. Though it would be nice if they were like "ah, whatever, here's the KQ IP for a reasonable price."