Have you seen United's fanbase? I think they're the most supported club in the world, or up there with Barca/Madrid. I think more kids worldwide would want his jersey if he was a United player than kids in the United States would want it if he played for a random MLS team. Stars do fill stadiums, but it's still very hard to convert kids to like soccer, which is all kids like in other countries. Jordan didn't make me like basketball, want to go to a basketball game or buy a basketball jersey. Same with Jeter and baseball and Brady and football. I don't think a player like Ronaldo coming would make people that don't like soccer go to games or buy jerseys, so his jerseys would be bought by people already soccer fans, which is a whole lot less people than are Manchester United fans.
Regardless, he will still be a top 3-5 player in the world at that time barring a serious injury. Can the MLS really get a player that good, while they could still offer their services to any team in the world?
Edit- I know it's hard to tell, but I just read online United's fanbase is around 600,000,000 people? Less than 2 million people watched the MLS cup final last year. I know not every Man U fan will buy a jersey, and more people than just watched the cup final will get a jersey. But you can't underestimate United's global appeal.