There’s no guarantee of having enough people, most importantly there’s no guarantee of having enough of the TOP athletes. Look what’s happened to baseball, kids aren’t playing it as kids so they aren’t playing it as adults either, it’s the reason for the big Latin invasion. In spite of all their efforts the NFL still isn’t big internationally, if they lose the kids they don’t have replacements lined up, the talent pool drops in both size and quality.
Football is likely to face a similar decline as top athletes migrate away from the sport. Some of them go to basketball, some go to soccer, some go to what used to be marginal sports in high school (volleyball, lacrosse, etc.), and some lose interest in athletics entirely.