Are you talking about soccer in the US or worldwide? Worldwide nothing comes close or will.
it's growing fast in the US but could be growing a lot faster. it's becoming more expensive for kids to play the game. Worldwide it's the "poor" sport because like you mentioned "All you need to play soccer is a ball, and for that reason alone, it will always be popular." Kids will use anything for a ball and goal and if they can't find anything they'll make a ball out of trash or play barefoot on dirt and rocks. Kids here do have it better than the kids mentioned. No kid here will play like that and also not many will have to. Also a lot of kids here would rather go play any of the three major sports instead of soccer as those sports are part of the culture here compared to soccer in other countries. Being a pro in any of the three sports here is big bucks
I found a comment that might help:
Played soccer for a long long time.
What he is referring to is the traveling & time investment (year-round) required by the institutionalized US system. Serious competitive club teams must both practice weekly & compete in tournaments usually 2-3 times a month. These tournaments are held all throughout the country. When I played I traveled to literally every state on the eastern half of the country; Detroit, Kansas City, Orlando, Atlanta, Maine, even on the national mall in DC.
On top of that there are league costs both indoor and outdoor, coach/management fees, the works. Soccer is expensive because its not as popular, the US is a pretty spread-out country, and you need to constantly travel long distances to compete (compared to the relatively densely populated Europe and S.A. cities, like Mexico City for example).
Ironically, the reason the US is inferior to most countries' soccer talent is precisely because soccer itself is not expensive at all, it's a cheap game to play - as you mentioned, you only need a makeshift ball and some sort of goal. Thus it is a perfect game for children in third world countries. To those kids it's practically the only option for sports - they dont have basketball hoops in their driveways, garages full of bats/balls/gloves/sticks, etc.
another problem is the US' approach to sports in general when combined with it being less popular;
- many children are often forced into a sport to occupy them, give them exercise, and possibly provide scholarship opportunites ... not because they like to play (re: the cliche piano lessons). soccer is most often the sport parents force on their kids because it is easily the most physically demanding sport - a sort of 'soccer will consume his energy & tire him out' attitude by the parents.
- there are no US soccer celebrities, soccer isn't flashy to kids
- schools do so very little to develop the sport compared to football, which is how football succeeds despite being a logistical nightmare sport to organize. high school & college football are the source of development, not private club teams.
- the national team has basically institutionalized the system to go through their manufactured channels (e.g. the Olympic Development Program [ODP]) which is a crock of **** that does nothing to help players, just filter talent. The end result is a bunch of selfish individual players forced to try to play as a team (which we all know doesn't work).