So you think there's more turnover between the top and bottom teams in leagues with promotion / relegation like the Premier League, Bundesliga, and La Liga, than there is in MLS??
San Jose won the Supporters Shield in 2012, they are second to last currently. DC United was one of the worst teams in MLS history in 2013, they are top of the eastern conference currently.
Do those pro/rel leagues have anywhere near the same number of teams capable of finishing anywhere from first to last over the last decade? Chivas was first in 2007 and RSL last. See also teams like Kansas City, New England, Columbus.... heck even LA was tied for last in 2006.
Toronto spent like a hundred million dollars on players and are spending that more than that on stadium improvements, when they have no incentive to do so according to you. Not sure why KC rebranded and improved. Or why RSL went from a joke to one of the best teams. Or why San Jose privately funded their own stadium. Or New England paid millions for Jones. Maybe those teams don't realize that they have no incentive to get better?
If anything, promotion / relegation encourages the teams at the bottom with no hope of winning trophies to play unattractive soccer hoping to squeak out a few wins and a bunch of draws to keep them from being relegated.
Washington Post: The unappealing moneyball of Crystal Palace and Hull City
Of course out of all the pro/rel fanboys in the US you won't see too many fans of Hull City or Leeds United or Kaiserslautern or Real Zaragoza I'm guessing... funny how that works out, Americans rooting for Arsenal and Liverpool and Bayern Munich and Real Madrid fully support promotion and relegation, and neither one applies to them!