My guess is that plane shifters eventually get memory-wiped and eventually die of old age at whatever plane they end up.

First, killing everybody may cause hassles both for Good gods (moral conundrums and such) and for Evil ones (less pawns for their schemes). You also need to check every plane for every living being. This doesn't seem practical when they already memory-wipe Outsiders, so why not extend it to all creatures? Much less trouble and less likely to start petty fights between gods for deaths and such.

Second, TDO has been building up power for (presumably) centuries, from many thousands of goblins and related races. If gods with more followers and whole pantheons backing them up (as Thor said here) can't survive the interim period, it's likely that no mortal can. Also, the gods are hyperbolic at everything, so the bureaucracy in deciding a new world and the time for each god to decide on something to create probably is as well.

If my theory is correct, it's quite sad for Kudzu, since he'll get memory-wiped and will never remember his father