I agree you cannot compare the national team's popularity in the biggest worldwide tournament held once every 4 years and start comparing popularity to the average NBA game. Not sure it is even fair to compare it to an NBA finals game - that is how rare the US playing in the World Cup is. It is just supply and demand. I expect the WC will give the MLS needle a nice boost, which had already been steadily growing.
Having said all that - I remember when the US lost to Iran in 1998. I was so angry, I had to blow off steam and went for a walk around the upper west side. No one on the street was aware a game had just been played, and certainly no one seemed to give a . There was zero atmosphere for it - no jerseys being worn, and very limited media coverage.
Today - it is all over every media outlet as the, or one of the, top stories. I watched the game in a packed bar on the promenade in Santa Monica, which was raucous from start to finish. As I walked out, there were disappointed faces and USA jerseys pouring out of every bar and restaurant. The change over the last 16 years is so profound that I think it is foolish for anyone to make any predictions about where it is heading.
It would be real interesting if the MLS has the guts to let NYC FC expand its payroll and go all Yankees on the MLS. I am not smart enough to know whether it would kill the league or not, but the idea of a world class NYC based club with deep pockets to attract intl superstar talent is intriguing. I know I am going to hear a lot of comparisons to the Cosmos of old, but the world has changed dramatically since that time - and having a champions-league type club in the US is the obvious next step. Not sure how it can happen however with the current MLS structure.