Have you seen some of the athletes we produce? Even in my tiny state alone, look at guys playing college football like Tyrann Mathieu (aka Honey Badger) and Trindon Holliday, these kids could boss the soccers if they had opportunity. Even Odell Beckham Jr was a club soccer player, but he quit because he'd have to move to another state to play at a club his level, which may be the norm in England but not in the US.
That's three world class American Football players from the same College Football team (like three from Barnsley, etc), who each could have easily dominated other sports.
I really doubt that they would have. Americans have this odd, parochial presumption that something like the "dream team" in basketball is the natural order of things - that Americans are inherently just better than anyone else at sports, and would be at footy too, if only they took it seriously.
But like I've said, the obverse is true - that this appearance of dominance comes from having been dominant in sports where there is little if any international competition. And if Europeans and South Americans and Africans even knew about led alone competed in sports like basketball or baseball or American football, this illusion would evaporate in a hurry. Against a global degree of competition (like in football), I doubt any of the people you mentioned would be household names - they might struggle even to hold a starting spot. Compared with making an impact in world football, American sports are an absolute breeze. Even strictly within the United States, the talent pool is not even close to being fully maximized - due to class and cultural factors, competitive athletes in the US are overwhelmingly African-American, which these days is probably not even 10% of the population. And being dominant at a small, provincial sport against such an already internally-limited talent pool does not automatically translate to anything on the world stage - it's really a minor accomplishment, in relative terms. It's like listening to Australians or Sri Lankans or Canadians insisting they'd win the world cup if only the LeBron of Aussie Rules or cricket or ice hockey had gone with footy.
Beyond the fallacy that raw athletics (which is what people here mean when they refer to LeBron, or... your NFLers) would automatically get you anywhere in football (soccer), where vision, awareness, and intelligence mean so much more, the presumption that the US naturally belongs alongside Brazil, Germany, Spain, France, Italy in football and would already be there but for the fact LeBron went for basketball instead is just absurd. Top American athletes don't even face a fraction of the competition that exists in world football. There is no doubt that there is room for improvement, but I don't really think it's because American kids choose other sports - for all the class restrictions, the number of youth soccer participants in the US already dwarfs most countries in Europe. It's just that they aren't very good, and are not going to get very good with American coaching. The US football system is not capable of producing top players, and just pumping more kids (or LeBron) into this system won't change anything. If Americans traveled like the rest of the world does, you'd have a better chance, but I doubt more than a handful of US kids will go to European academies, where they can earn real money, though those few who do will probably all be the top US players in a few years. But even then, there's no reason to think the ceiling for US soccer is any higher than it is for countries like, say, Mexico, or Turkey, or Japan. You're just not as divinely good at sport as you think you are, and if it wasn't for CONCACAF, you'd barely ever qualify.
