Really? It isn't just the heirs who've complained about this; the creators did during their lifetimes. WB had to be publicly shamed into giving them pensions.
The heirs should take WB for every penny they're legally entitled to, IMAO. No sympathy for the corporation here.
Oh, and what they're "legally entitled to" is what we have courts to determine
. Anyone who announces on a BBS that they know "what the law is" and what's "fair" is just blowing smoke.