Well additionally, meat is usually best tasting when the animal is more or less culled during the prime of their life, so not only would a young person have to be prescient enough to write a will, but be in the very small percentage of young people who also desire to be eaten.
This argument, even from a cold logical standpoint, still strikes me as rather nonsensical, as anyone who might actually desire this wjo had thought it through, would have had extensive life experience to come to such a conclusion. This would result in an elderly person essentially saying, “please eat my tough not succulent flesh after I pass away.”