Why is Bill Murray a 'shitlord' for not wanting to do a sequel? If anything, his reluctance to do any script thrown at him for a quick buck or needless sequels to some of his classics is something I admire about him. I too thought Ghostbusters 2 was fun enough as a kid, but let's not start implying that it was particularly inspired.
These days, I would say Bill Murray is more famous for his 'second' career, with his Wes Anderson movies, and Lost in Translation and the like. He moved on, frankly to something much better. Something, if we're being totally honest here, none of his Ghostbusters co-stars managed to do. If he didn't believe any of the Ghostbusters scripts were anything more than needless nostalgia and he didn't feel good about doing it just to appease some fanboys, I'm totally fine with that. He doesn't owe us anything.
And to be fair, he did sign on for the last script option before Ramis died, which had him dying early on and appearing as a ghost here and there. And he still has a cameo in the new movie, I think. A Ghostbusters 3 with the old cast would just be awkward. Dan Aykroyd got super weird in his later years, and I don't think he has it in him to act that role anymore.
