Q.: What's so bad about believing people go to heaven?
A.: For openers, such a believe is incorrect and has no basis in observable reality. It is an extraordinary claim for which no extraordinary proof is offered. The fact is that there is no such thing as a "spirit realm" populated by any kind of gods, angels or demons. There is no soul and there is no heaven for a soul to go to. These things are the inventions of chimpanzee imagination seeking desperately for a meaning for their lives. On a universal scale, our lives are as meaningful to the universe as the life of a spider, axolotl or fossa.
Secondly, such a belief encourages the believer to view this life as a "dress rehearsal" for something better that will come along as soon as they finish with this nasty trail of tears. Such a belief encourages people to tolerate evil because they figure that evil will be handled in the "world to come."
It makes trusting simple souls the dupes of narcissistic liars and con men.
It is the desperate refuge of ignorant shallow thinkers.
THAT is what is so bad about believing people go to heaven.