I'm shocked that you think Manchester United will be unable to attract top players in the future because they made one young goalkeeper honour his own contract. You should have more faith in this club. The great characters of this clubs history would be more likely to criticise the player who wants to leave than judge the club poorly in such a situation.
When was the last time we signed "the next Neymar or Messi"? How have all these wonderful South Americans performed when they've arrived at Manchester United? We signed Cristiano Ronaldo, raised him and coached him into one of the worlds greatest players only for him to leave and spend his peak years at another club. We spent the money we received for him on Antonio Valencia.
There are plenty of brilliant players available at home or nearby, in France, Belgium, Holland, Germany, Italy and in Eastern Europe who have wonderful careers without needing to fulfill this strange fetish of joining that circus in Madrid. We've had a more than successful history without pandering to Spanish and South American passersby and we'll be successful again, with or without them.
The club isn't infallible when it comes to the power of players and agents in the modern game. Just because they don't take the action they should, doesn't mean it's the wrong action.