It's not like players by this point don't already have =cold from AMW.
True, intrinsic immunity is always better but it's not really such a huge advantage.
Without it, you simply have to switch armors for different locations - powerful fire and frost monsters together appear very rarely to require the PC to have both immunities to survive so I don't see much of a problem here.
Also, there's plenty of those lizards spawning around FGJC and IQD already to warrant lots of corpses anyway.