Gladwell is not a scientist. He was summarizing for laymen what researchers have found. The 10,000 hours is not a mean and not a calculated number of any kind. It is an approximation in a theory. I think Gladwell did an excellent job of characterizing the body of information. I think, however, that a lot of people have mischaracterized what Gladwell wrote, people I suspect who were commenting about what Gladwell wrote without the benefit of actually reading what he wrote. So few people read books any more.
Gladwell also looked at the super elite. For ex, Michael Jordan, The Beatles are two that come to mind; and these are not analogous to the German National Team players. To become Jordan, you'll most likely have to put in 10k hours. To be a role player in the NBA you may only have to put in 6k. If Gladwell had included soccer players in Outliers he would have looked at the "Jordans of soccer" -- Pele, Maradona, Messi (not the German National team). I would bet that the creme de la creme of soccer put in more than 10k hours. Certainly Messi did as he started very young and played a ton all through his youth and teen years. I can't think of any super elite soccer player who hasn't put in 10k hours +. Again, Bradley/Donovan/etc are not super elite; I'm talking about Ronaldo/Messi, level players (let's say top 1% of players in European leagues).
I read Outliers when it first came out. Loved the idea, pitched it to my players and parents as fact. But in the intervening decade, many researchers have come out and said 10,000 hours is not iron clad—there's a lot of variability in the amount of time required to achieve mastery. Not sure why that's so controversial.
Arguably, the current research is pointing toward that soccer and tennis probably pushed her closer to her true potential in basketball rather than away from it. I don't believe the research covers overlapping sports—that's a whole different area of study.
I don't believe its a different subject at all, and I believe the research on musicians covered it. All formally trained musicians play multiple instruments. Keyboard is required along with music theory classes. A lot of amateurs play multiple instruments too.
Another data point https://bjsm.bmj.com/content/early/2019/11/18/bjsports-2019-101365.full Impact of youth sports specialisation on career and task-specific athletic performance: a systematic review following the American Medical Society for Sports Medicine (AMSSM) Collaborative Research Network’s 2019 Youth Early Sport Specialisation Summit Results Twenty-two articles were included in the final review; 15 addressed career performance outcomes and 7 considered task performance outcomes. All identified studies were cross-sectional or retrospective in design. The proportion of elite athletes who specialised early ranged between 7% and 85%, depending on sport and definition of specialisation. Elite athletes often specialised between the ages of 14 and 15 compared with their non-elite or semi-elite peers who typically specialised prior to 13 years. In addition, neuromuscular control, anterior reach asymmetry and physical task outcomes did not differ by specialisation status. Conclusion The volume and methodological rigour of published research in this field are limited. Our review suggests that YSS is not required to achieve success at elite levels. YSS also does not appear to improve task-related performance (eg, anterior reach, neuromuscular control) outcomes for specialised athletes when compared with non-specialised athletes during childhood and adolescence.
I think it is commonly thought that multi-sport athletes have a better tactical understanding of soccer. Also I think it is commonly understood by coaches exposed to multiple sports, that, of all sports, soccer places the broadest range of challenges in the physical aspect of performance. I think playing multiple sports "builds" a better athlete in the physical and tactical aspects. I know that my own experience supports that in physical development. For instance my first step quickness came from competitive singles tennis, as does my appreciation for positional play. You might not think tennis has much to offer, but it does. It isn't just basketball that helps.