((Knows joke but gonna pursue endlessly)) Chaos Spawn die within minutes or hours of their transformation, being killed by their former followers, slain in battle or expiring as their impossibly mutated bodies give out under the strain.
Spawns don't get to just die, they must wallow in their folly until slain on the field. The warp has a twisted sense of humor. -Smacktually, there is a story of a nurgle spawnling that the guy basically just turned into a fat toad that ate whatever it could, this amused Nurgle greatly and he made it a daemon prince. So, at least one chaos god canoically has a sense of humor.
Qoute from lexicanum. Edit: only foulspawn has lived for some time Still dont know what it is other than it keeps you on school
Clearly some spawn would have dysfunctioning organs, but this would exist in areas away from the warp. Say some cultist on a mid Imperial planet pulled too much power from the gods, and just collapsed into a heaving mass of misbetotten flesh, in areas of high warp exposure, the lack of comprehensibility that such a creature can still function is irrelevant, because the mindless presence traped within will keep mutating until it finds some semblance of function. and Are both equally possible spawndom forms, the latter would likily had been a tech adept who fused into a mindless murder machine, and the centar a beastile spawn, keeping an animal-like semblance of mind. You know, or a mush of miscellaneous organs.