Toronto FC - Justin Morrow - Jonathan Osorio - looking down

Toronto FC's long-dwindling playoff hopes finally, officially died with Saturday's 2-1 home loss to the Vancouver Whitecaps, a setback that entered the Reds into a small but unhappy club. 

The 2017 MLS Cup winners are just the fourth team in MLS history to win the league championship, then fail to qualify for the following year's postseason. In this regard they join the 2000 D.C. United squad, the 2006 LA Galaxy and the 2016 Portland Timbers.

Winning a title is one thing; defending it is quite another. In MLS specifically, the league's parity, the difficulty of keeping winning rosters together and the additional work and stress brought on by additional competitions like Concacaf Champions League are the factors most often discussed as culprits in these letdowns.

What do you believe is the biggest challenge to reigning champs? Share your thoughts in the comments section.