Here's one way to look at how difficult it is to build a dominating dynasty in the modern sports world, where free agency, huge budgets, and overall competition has changed everything.
Take all the champion MLB teams after the Yankees won their last world series in 2000: 15.
Take all the champion NFL teams after the Patriots repeated in 2004: 11.
Take all the champion NBA teams after the Lakers three-peated in 2002: 13.
Take all the champion NHL teams after the Red Wings repeated in 1998: 17.
So, that's 56 teams that have won the championship in all four major sports leagues, since arguably the death of each league's last "dynasty".
Of 56 teams, all of whom are presumably a top contender and have the talent to repeat as champions, only the 2008-09 Los Angeles Lakers have repeated as a champion. 1 out of 56, less than 2%.
That's downright incredible. Particularly in the NBA, where in the past it was rare that a team didn't win it all the next season, but in all leagues. Teams have won 3 out of 6, 2 out of 4, etc., but just the fact that no one can repeat two years in a row goes to show how much more difficult it is now to stay on top. The huge majority of these teams didn't even make it back to the finals the next season.