The fan theory video below is from The Film Theorists YouTube channel. Basically, the fan theory is that shortly after everything that happened in the 1966 Dr. Seuss' How the Grinch Stole Christmas! animated holiday special the Grinch died. The Grinch, by the way, is voiced in the special by Boris Karloff which seems rather fitting for this theory.
Biology is not my strongest science but based on the fan theory video I tend to agree that it can be deduced that the Grinch had chronic fatigue syndrome. It also seems that after the Grinch spent Christmas Eve breaking and entering into as many buildings in Whoville (not be be confused with Doctor Whoville) as the Grinch could he had a massive heart attack on the top of Mount Crumpit. Since the animated special doesn't show the Grinch receiving medical attention it can be assumed based on the fan theory that one or more ruptured heart valves lead to the Grinch dying sometime shortly after the 1966 holiday special ends.
I may not be as strong in biology as other sciences to have enough confidence in fully supporting this fan theory but I can examine how it fits a timeline. According to Wikipedia Halloween Is Grinch Night was a prequel to the 1966 animated special. I can't find any definitive references to the prequel or sequel status of The Grinch Grinches the Cat in the Hat. However, the Cat in the Hat crossover begins with the Grinch waking up and reciting the "Grinch's Oath" while looking at his reflection in a mirror:
“A Grinch is unhelpful, unfriendly, unkind, with ungracious thoughts, in an unhealthy mind. A Grinch is uncheerful, uncouth, and unclean. Now say this together: I'M FRIGHTFULLY MEAN! My eyes are both shifty, my fingers are thrifty. My mouth will not smile, not half of an inch! I'm a Grinch! I am a Grinch. I'M A GRINCH!”
If the Grinch did permanently change his ways in Dr. Seuss' How the Grinch Stole Christmas! then the animated Dr. Seuss crossover should also be a prequel.
Based on the fan theory video and the timeline it seems that the seemingly functioally extinct Grinch did die sometime after his attempt to steal Christmas. That's karma I guess.
