I can give you a view of what things might be in 10 years. My son is a 17 year old high school junior attending a school that competes in the highest classification in our state. He splits his time between basketball and tennis, and has for the past few years. Tennis is the sport he will play in college, but I think basketball is his favorite. Of course that is probably because he has all his school buddies on the team, and has 1,500 people in the stands screaming at the games he plays in. Tennis is training with dad (who he still can't beat), and playing many many many usta tournaments with maybe 5 or 6 people watching his match. As others have said, basketball is a great supplement to tennis. We train for tennis at night after high school basketball practice, and weekends. We get to hit 5 days per week, only skipping game days. at 5ft 10, he is an average athlete on his basketball team, with 6ft 5 crazy quick kids going in for slams. But when he takes the tennis court, he seldom faces someone as quick as himself. The downside is that he spends less time on tennis that most of the top regional tennis talent, and less time on basketball than the same from that sport. Injuries can also cause problems, such as a missed tennis training session Monday while he was being taken to the hospital to have his eye sewn up from a practice injury. I suspect he would have more interest in tennis if he had good friends to train with, but he is the only serious tennis player at his school. He is enjoying both sports now, and if he does get good enough to play college tennis, all his basketball training will help him as an athlete, toughness, training hard, and in other ways. If a kid wants to play another sport along with tennis, basketball would be high on the list of options. I can also tell you as a former college tennis player, and someone who played most sports growing up, those tennis players that just played tennis were not hard to spot. They had tennis skill, but were not very athletic when we did other activites for fun.
Kalamazoo may be the only USTA national that some matches are played in a stadium with a few hundred seats. But he would get that many or more people cheering for him in every basketball game