Los Angeles County District Attorney George Gascón will leave office Monday after being defeated for reelection by 20 points. He is one of several progressive prosecutors to lose their jobs on the West Coast over the past two years, from San Francisco and Oakland’s Alameda County to Portland and Seattle. But this is the biggest blow yet to their movement: Not only has Mr. Gascón long been considered “the godfather” of progressive prosecution, Los Angeles County — with nearly 10 million residents — is by itself more populous than all but 10 states.