Sure, this is true for many fans.
But I'd argue that *most* STHs of any NFL, NBA or MLB team would watch the ultimate game of the whole season even if their team didn't make it. Most of us will probably watch the super bowl this year (including me, even though I hate the NFL) - add to this, the context that as MLS fans you're probably painfully aware of the fact our TV ratings are pretty shit and most MLS teams operate in the red and therefore you feel obliged to support the league wherever possible...
I dunno.
The NFL isn't far to compare because they are the beasts of ratings, gambling, fantasy, and interest in this country. People will still watch two horrible teams on Monday Night Football for no other reason than they have office pools, fantasy games and money on the line. The NFL is in a league of its own right now. Even the NFL Draft, which is just college players getting drafted has become a 3 day prime time event that garners higher ratings than the playoffs in the other major sports.
I think we need to compare MLS more to MLB, NHL and the NBA. In those leagues it's fair to say (especially in baseball and hockey) that most fans stop caring once their teams are out of it. Even the NBA hopes for big market teams to make the finals. The Superbowl is basically matchup proof, the entire country will watch regardless of who is in the game. As for me, it was hard to watch MLS Cup because my team just got elimated the week before and it hurt to watch. I flipped here and there between NFL and MLS Cup but only to see what was going on. I still think going head to head on Sundays with the NFL is not the best idea when you're trying to increase ratings and interest. This is opening a whole other discussion with the schedule and FIFA calendar but I'd prefer more of a yearlong schedule with a winter break (like Bundesliga).