On my reread I found the following excerpt in which Prothall argues that Drool has already found the Illearth Stone.
High Lord Prothall wrote:
The evidence of power is unmistakable. We know that Drool has the Staff of Law. But the Staff is not a neutral tool. It was carved from the One Tree as a servant of the Earth and the Earth's Law. Yet all that has occurred is unnatural, wrong. Can you conceive the strength of will which could corrupt the Staff even enough to warp one bird? Well, perhaps madness gives Drool that will. Or perhaps the Despiser now controls the Staff. But consider - birthing the three-winged bird is the smallest of these ill feats. At his peak in the former age, Lord Foul did not dare attack the Wraiths. And as for the desecrated moon - only the darkest and most terrible of ancient prophecies bespeak of such matters.
Considering what Linden has done in Runes and at what effort, I think it sounds like the old Staff was much stricter about the Law than the new one. Particularly note the words "corrupt the Staff". That sounds pretty ominous. As I have said before, I think the runes protected the old Staff from corruption. And since the Law and the Staff are interconnected, I think Linden may end up warping the whole structure of Law itself by proxy. Think about the Staff of Law as a force for evil. If the good guys keep it they are allowing evil to prosper while endangering themselves. If they destroy the Staff, they are weakening the Law even more and making Lord Foul's job easier. That sort of dilemma sounds like the kind of thing that SRD would use.
Also, the raw materials of the new Staff are suspect. I have written about Findail and the Shadow upon the Elohim (Sunbane), and Vain might have something too. Perhaps the ur-viles as un-Lawful creatures didn't like the current Law that much, and decided it would be better to put a hidden weakness in Vain so that it would be possible to make the Law a little more accommodating and perhaps even warp the Law enough for an ur-vile to wield the Staff in safety.
Even the Second Chronicles had aliantha. I have a feeling that yet another previously untouchable sacred cow is going to get it, and the Law's corruption will turn aliantha poisonous. And I think Lord Foul will get the corrupted Staff, since that's so very fitting from the Gnostic perspective and because SRD needs to get something worse than the Sunbane of old. The logic of the plot calls for a Hell on Earth. Runes is just warm-up.