The World Cup gave MLS unprecedented momentum. Keeping it will depend less on signing aging icons than convincing America’s best young players to stay “Thanks World, we’ll take it from here,” is Major League Soccer’s message to the American public. Yet two moves in the last week were emblematic of the issues the league faces in taking on the mantle of North America’s hosting of the game’s grandest stage. A rising, under-the-radar homegrown player left for Europe, and a veteran with immense pedi…