so your concept of the sport in general decline is not many kids on your kids' elementary school team?
ok... I'll bite.
at a guess, lower level rec soccer and something like 3rd grade soccer might be on a decline. I'd bet that the kids who actually play the sport aren't bothering with elementary school soccer because the quality of play and coaching is low and it likely interferes with their club team schedule. the club soccer kids might even be playing other school sports instead just to mix it up.
with the pro game leaking down into the youth game with pro academies in place all over the country for kids- I think the game has become less "send the kids out for something to do by the soccer-moms" and more, my kid has genuine ability and might go pro some day.
I'll leave it to the boots on the ground guys in here, but I think the youth game has become much more serious and stratified. where non-athlete or non-serious kids might have kept playing into or past middle school, I have a sense the level ramps up so much the bottom end might have fallen away a bit.
soccer on the decline in general is categorically wrong across pretty much every metric (besides your dislike of the sport).
average wage of MLS guys is now pro-athlete levels instead of working at the hardware store levels, and kids are routinely bypassing the old route of college ball to go straight to the pros here in the US and abroad. tv rights are setting records, as are intro costs to buy into the league and overseas ownership. the sport is on TV and internet 24/7and I would guess more kids play the sport- as it was in the 80s- than any other sport. IIRC more kids played back then than baseball and football combined.