It could be argued that American Football is franchised and lacks relegation or promotion, and that game has flourished in spite of this.
But American football has a salary cap and a draft and part of the appeal of the game is that fans know there's a chance that any team might win even if it takes a few years of rebuilding.
Soccer fails in the US because the 'good' athletes who play it give up as soon as they're old enough to play basketball/football/baseball more seriously since there's far more money and opportunity in those sports than in the MLS. You really have to love the game to play it for a living.
The other problem is that it's not particularly exciting to the casual fan. There's a heck of a lot of kicking the ball around midfield without much going on. While in soccer-oriented countries, this is seen as a careful skilled buildup (yawn) and there are subtleties involved, US fans are more likely to see the subtleties in how a baseball is pitched or how the formations line up in American football; the things that the Brits just don't get since they don't see them.
Soccer, unless played at a high level where both teams are playing to win, isn't exciting enough. It's flourished because it's cheap to play at grassroots (all you need is a ball and a couple of shirts to use as goalposts) and relatively easy for anyone to pick up and have a kickabout. The strategies and tactics don't begin to compare to the complexity that top NFL/NCAA teams have to use in order to succeed - which is both a good and bad thing. Heck, I've been watching American football for the better part of 20 years and I still don't 'see' everything when the analysts break it down.
I used to watch UK football, I still watch the odd game in the World Cup and I'm happy to appreciate the highlights of great goals or stunning footwork but there's not enough of that in the average game to make me watch all the way through. And I'd guess many American football fans would feel similarly.
Back on topic again though. I don't think Beckham going will make much of a difference to the popularity of the game although those who are current fans will enjoy it. I just hope that in the presence of real Hollywood stars, Posh gets her current sense of the Beckhams celetbrity bubble slightly deflated.