The man who is a pessimist before 48 knows too much; if he is an optimist after it he knows too little. (Mark Twain)
Pessimism does one unforgivable thing, it withdraws libidinal energy from the future. It questions whether continuation is justified. That alone destabilizes human systems built on growth, reproduction, productivity, debt, and obedience. A pessimist is not dangerous because they are sad; they are dangerous because they do not automatically feed the next cycle.
This is why pessimism is treated as immoral rather than mistaken. It interrupts the emotional fuel supply. It makes people hesitate instead of reproducing, consuming, voting, smiling, scheduling, and complying. And people cannot tolerate hesitation. They require affective forward motion, even if empty, even if false.