DENVER—Developer Asher Luzzatto stared out at the snow-capped Rocky Mountains one recent April afternoon, admiring the postcard view from the 29th floor of his recently acquired office tower. Asher Luzzatto paid just $5.25 million for this two-building complex in downtown Denver. Then he turned his gaze lower, to the streets surrounding the downtown Denver building, where he glimpsed a less-inspiring sight: vacant store fronts, empty office buildings with darkened floors, and deserted streets. The property, one of two office towers known as the Energy Center, is a testament to downtown Denver’s rapid decline. Luzzatto picked up the 785,000 square foot complex late last year for a little more than $5 million—97% below what a previous owner paid for it in 2013. America’s downtowns are suffering a crisis. Office work has migrated into the suburbs, leaving abandoned buildings and blighted conditions behind. Central business districts from St. Louis to Dallas and Portland, Ore., are fighting to escape a death spiral. Luzzatto has a plan to fix these downtown catastrophes. He is starting in Denver, where nearly …