why heaven and hell can't exist
Wrap your head around this one.
Hell sucks for eternity, that can't be. If you were in hell in an eternal state, the fact that it is bad in comparison to say life on earth would be negated by the simple premise that you would get used to it. Therefore it would become normal just like your existence now.
Same goes for heaven, wow it would be great until you got used to it, the pleasures of heaven become negated.
Without heaven and hell religions have no purpose, because they sell the antidote to the fear they create.
Death without heaven or hell is just an ending, it neither sucks or is good, there's nothing to worry about. The people who make their living selling fear need this intangible. Without it they have nothing.