Defeatism is not realistic. If The Founders had been “realistic” we’d still be a colony.
Realism is what prevents a 110 lb skinny kid from fighting a 300 lb martial arts master. It's what prevents a little country with no army from attacking a big country with a big army. It's why we have divisions in college football, weight classes in boxing, sports leagues split between men and women, etc. When the score of a football game is 45-0 in the last 5 minutes of the 4th quarter, sometimes you face reality, pull your starters and plan for how to move forward to the next match.
It's okay to reach for the stars, but at various points in life reality sets in and one sometimes has to adjust for it.