"Back in the day" which was the 1980's, there were players in HS that were 3 Sport Stars.
NOBODY even thought about being a single sport player to "specialize" in a sport.
Baseball made me a much better football player.
In baseball, you need to THINK AHEAD about what you are going to do BEFORE the ball is hit at you. If the ball is hit to my right, I do this. To my left, I do that. Ground ball right at me, where do I throw it? Cut off man, or sling it to 3rd base?
It's amazing how sports cross over, and you don't get the burn out from only doing the same sport over and over.
Also, you get exposed to different coaches, different styles of coaching, different drills, different strategies, ect. You become a better all around athlete IMHO.