Whenever a talented MLS player is transfered to a European league, there is always someone who starts complaining, that MLS will never become a top tier league, if they sell their biggest players.
This idea is partially true, but mostly wrong.
MLS of today is a developmental league. Some people might oppose to that, but the way soccer has developed it is really only La Liga, the Premier League and perhaps the Bundesliga, that cannot be described that way. Most of the best players in Ligue 1, Serie A, the Eredivisie leave for bigger and better leagues and I'm sure thats company MLS and its fans wouldn't mind being a part of.
The thing about players is that most of them are ambitious people. They want to be the best and play with the best, and when you as a club want to sign a player you need to convince them that along of an acceptable salary you can help them achieve that goal. MLS is a good league yes, but there are plenty of leagues, where the quality is higher and the salary is better. Consequently, what MLS must do to attract talent, is tell the players that what it can offer them, is a good league to develop in and when the right offer comes from a foreign club, they won't stand in the way (Unlike the Clint Mathis to Bayern München situation). If MLS and its clubs has a reputation of standing in the way of the players getting to bigger clubs, it will become harder for them to attract the talented players, both foreign and domestic, and this will have a negative impact on the league.
Obviously MLS shouldn't sell any asset at a discount rate, and big efforts should be made at convincing players to stay, but as we saw with Clint Mathis, there are also big downsides to forcing a player to stay, that saw a chance to move on.
A more contemporary example: If Fredy Montero decides that he wants to go to a top European league and a club fields a good offer, MLS should accept it. Yes, MLS would lose a top player, but they would earn a good buck and it would send the signal to young South Americans like Montero that MLS is a good, stable league to develop in, in order to get recruited by a top European team and being that stepping stone will help MLS increase its quality as more talented players go there to be spotted.