What he really wants is to deny as many blacks as possible the education, training, discipline, experience, leadership, and earned respect available in the military.
IIRC, Rangel is a Korean War veteran who is, of course, black and has a largely black and Hispanic constituency. Ironically, one of the consequences of his proposal will be, as you say, to deny more young people of minority background the opportunities available in the military.
This mandatory national service scheme, taken as a whole, smacks of similar schemes common in dictatorships throughout history all around the world. It flies in the face of the value of freedom of opportunity that has been essential in America's rise to the greatest nation on the planet.