How often do teams like the Yankees, Red Sox, and Dodgers make trades like that? Pretty much never.
Big market, World Series contending teams don't need to weaken this year's team to get players who might help in 3-5 years when the team is not guaranteed to be a contender. They try to win the World Series this year. Future seasons are secondary. Tampa's and Oakland's front offices may need to run that way, but the Cubs should not be looking to marginally improve their farm system by taking a few wins away from the current team.