I am going to take the other side of it.
My kid played a lot of different things when he was really young – baseball, gymnastics, track, basketball, etc. (But, no football.) As he got a little older, it settled on baseball and basketball. But, by the time he got to 8th grade, he decided to lock into baseball only.
He goes into freshmen year at HS and everyone wants him – the baseball coach, the basketball coach (who knew him from summer camps) and the football coach (even though he never played football but because he was known as being a good athlete.)
He decides – I want to lock in on baseball and only train for baseball because I want to make varsity as a freshman and be a starter. (And, in the end, he did.)
Meanwhile, all his grade-mates went out for every team. Some of his middle school baseball teammates played freshmen basketball and football. They went to every practice, every day, and went to every game – often just sitting the bench on game days. It was a whole lot of time “being a multi-sport athlete” and not really playing much in games.
Now it’s baseball season and my son is the starter on varsity as a freshman (very rare in our school) and the other kids are all on the freshmen baseball team. The kids and the parents are resentful of my kid being on varsity. And, my answer them all is: While your kid was going to football and basketball practice EVERYDAY and then sitting the bench at the games, my kid was in the gym lifting and in the cage hitting. He could have done the same thing as your kid just to say: I “play” all sports. But, rather than do that just for show, he focused on the sport where he had an interest and worked hard at it.
Now – this isn’t for the stud who is a stud at every sport that he plays. That kid should play them all. But, if a kid is only interested in one and doesn’t want to play others just for the sake of being called a multi-sport athlete, then he shouldn’t be dinged for just playing baseball.