> Pessimism will never be as motivating as telling people about the bright future ahead if they <xyz>

But this is a false dichotomy. There is no reason to tell people that they must do anything unless there is something to dread.

Example:

Assume that leading AI experts recommend halting advancement of super-human AI due to the danger of hostile AI.

"If you pass this law to prevent the further advancement of super-human AI, then the singularity will not reshape the world."

This does not mention any danger, and thus implies that if they do not prevent the advancement of super-human AI, there will likewise also be good consequences.