Having to deal with these ignorant casual tennis fans gets a little old because I have to deal with people that talk these things over at Hooters drinking beer and eating chicken wings after a league tennis match and they think they KNOW that if would-be football, basketball, hockey, etc. players picked up tennis, the US would become world power in tennis again.
Why don't you guys read up on what it took for the current top-5 men got there? You will get the answer. It requires A LOT more than just being a good athlete to be a top tennis player.
Conversely, if you are a decent athlete, you can at least become 6.0-6.5 player by being exposed to the same level of high quality coaching/training/competition, etc. that most of these pros have received in their lives. That, of course, means that you would have to start playing/training 3 to 5 hours of tennis starting 6,7, or 8 and continuing all the way into the adulthood.
This would also mean that you may have to be home schooled, or go to a special school at night (like they do in Spain). Your family may need to move to an area where you would have top quality coaching and other good players to train with. Your family would have to deal with literally hundreds of thousands of dollars of cost in coaching/lessons, traveling, etc.
If you have the right talent and are mentally and physically tough and durable enough to go through the training, perform (focus) well in competition, and, you and your family are lucky and resourceful enough to find top quality coaching suitable for the right stage of your development, you might make it as a touring pro, and perhaps more.
My point again here is that it takes A LOT more than just being a world class athlete to be a top pro. Yes, it helps to have a larger pool of good athletes but it is not gonna get you far if you don't have top quality coaching/competition and the resources to obtain them.
In case of American tennis, my contention all along has been that poor quality/outdated method of coaching has a lot to do with the recent decline. Just a few top draw coaches may make a big difference. Plus, as I have mentioned, don't even mention athletes that excel at other sports that have little to do with tennis. There are probably millions people that are more athletic than some of the top tennis players, but it doesn't mean much here.