According to NPD as of 5 months ago, the amount of Nintendo Switch fans which also own a console from another major console manufacturer is around 70% in the U.S. Obviously with more Switch units being purchased since then, that statistic has probably changed. However, it does highlight a key point. While it would be easy to downplay the Switch by using this statistic to show that it is mostly purchased as a secondary console, it shows that the Nintendo Switch shares largely the same demographic as the PS4 or Xbox One. It's hard to discern how much potential sales it could or couldn't be taking away from other consoles, but the point is that when the excuse is made that the Switch isn't competing against the PS4 or Xbox One because the userbase is completely different, the reality is very different from the argument being proposed.
Let's just be honest, the age range for the PS4/Xbox One and the Switch is largely the same. Nintendo might have a higher percentage of the extremely young market, like the 4-7 year olds, but with the gaming industry being bigger and gaming becoming more and more normalized ... I think that something like COD and Grand Theft Auto appeals to kids just as much as Mario and Zelda. When I moved into my current neighborhood 10 years ago, my neighbor had a kid who was like 4-6. He didn't play Nintendo games, all he played was Call of Duty and Battlefield Bad Company 2. Anecdotal, yes, but I think we're starting to see M rated games become more and more adopted by the 8+ crowd.