MLS is a growing soccer league in North America. So when it rejected a media rights offer that would've given it six years before it came to the market, it shocked many people. After all, this is a league that's experiencing rapid growth. However, the league would've had to put in a relegation/promotion system in place.
On the surface, it seems like a good idea to put such a system in place. Again, the league is growing with bidders from various cities wanting in. However, North America isn't as deep of a soccer pool as South America and Europe. MLS hasn't been able to lure in the top young international soccer stars that leagues like the Premier League and La Liga have been proven to do time and time again.
Now, relegation is a staple amongst many international soccer leagues around the world. However, many of these clubs are rich and able to lure in top players with studious players that would make MLS jealous.
Will MLS grow to the point of being able to compete with such leagues? Eventually. That would depend on how MLS markets itself to international players. However, relegation will have to wait until the league has significant growth and can lure in top players.