But the U.S. experience directly contradicts this. No country in the world has as many kids registered in youth leagues as the U.S. (not even close as a matter of fact), yet MLS and the NASL before it have been waiting for decades for those kids to grow up and become soccer fans, and it's never happened. Indeed, MLS has now all but ditched the strategy of trying to appeal to youth soccer players and their families, and is now in its more successful markets appealing to rowdy 20-and 30-year-olds with disposable income.
If you want to consider all the kids in the U.S. who haven't played in competitive leagues but have played soccer in school gym classes, you'll probably get to near saturation, but pro soccer still struggles comparitively in the U.S. Whereas pro football is a behemoth - but how many people have played American football of any kind?
The number of current and former competitive soccer players in Europe is dwarfed by the overall population, but soccer is undeniably the number one spectator sport by a wide margin.
I would say viewership of professional sports is almost certainly based overwhelmingly on cultural factors rather than on experience playing the sport.
But cultural factors, and the reason why a sport becomes embedded in the public consciousness of a nation in the first place, are largely influenced by accessibility more than the entertainment value and fast pace of said sport.
The US is different from most countries in that soccer didn't become pro early enough, while at the same time the US were a very precocious country in the development of pro sports.
For decades, soccer was a sport associated with European immigrants, and largely confined to amateur or semi-pro "ethnic" metropolitan leagues.
At the same time, WASP Americans already had an accessible, professionalized grassroot sport of their own, baseball.
Slow paced, unspectacular, yet cheap and easy to pick up, baseball was America's soccer.