I don't know... MLS has seen a huge change in the last few years that I never thought it would get. Look at the Pacific Northwest! Not to mention we may be getting Older guys on the down part of their career, but they still get the league attention and more people watching as a result. I also think plenty of players have gone from MLS to Europe and done very well for themselves. Howard, Keller, Guzan, Reyna, Dempsey, Donovan, Holden, Onyewu, Spector, and Bradley. No none have set the world on fire but i don't think that is MLS's fault. Like you said our best athletes choose other sports due to money and popularity.
The biggest issue is getting better athletes to choose Soccer over the other sports. I have a feeling we will start seeing that with the new concussion problems and with parents allowing less and less kids to play football. i know that when I have kids I am going to guide them to any sport or activity besides football as this reason.
Overall MLS has grown and came from a place where it looked like the league was about to fail to an actual profitable situation. But like you said it is about good athletes choosing to play, so that doesn't really fall on MLS. Besides getting Star Players on their decline that can still show something may be the best chance for kids to want to choose soccer as a result of getting to see them play in person!