ruleslawyer said:2) The "why don't the gods just kill the archfiends and take over?" argument that's advanced as a metasetting problem. This, incidentally, was constantly raised in Dragon magazine as early as I can remember. Gygax's solution was to just move all the deities out of the Nine Hells; the Abyss is plenty big enough for gods and demon princes alike.
Yet he gave Hextor the power to enter the Nine Hells at will. All deities could do so, actually, so that was really no solution. Unless the archdevils had the ability to deport troublesome deities under their own power (as Planescape implied they did) or unless an overdeity existed to prevent archfiends and deities from fighting, there was really nothing preventing deities from taking over the Hells other than DM fiat.
Gygax moved all the deities out of the Abyss as well. Note that none of the gods of the Greyhawk pantheon live there.