I will grant you that a mission is not for every one and some would be better off in college or elsewhere. However maybe that kid wants something different than you in life. Did that kid return from his mission to attend a college?
As for a net positive. That man dedicated, on his and his families dime, two years of his life to help others and spread something that has brought him profound joy in his life. I only wish more kids had that level of dedication, willingness to serve others and sense of responsibility when they were 19 (now 18).
To be honest I see them both as net positives for society and him. It is not like he turned down college to join the circus. He is dedicating two years to his fellow man. That is a deeply profound thing.
As a man that went on a mission.. what made you go?
When I was of that age, or round abouts, the big line was "Every able young man should go". Everyone that I knew my age was expected to go. Growing up, people looked at those who didn't go differently.. negatively. Even if it was for a good reason like they had a scholarship, or the family just didn't have the money to support that(sad, but it's a good reason not to go). The pressure was on.. Your bishop was after you from the age of 16 on. It actually started at 14 for me.
There's always the expectation and social push to get you to go to church. Especially as a young man.. they needed someone to bless/serve the sacrament. But at 16, every time I showed my face at a function of any time, "You ready for your mission?" was the first thing on someone's lips. At 18 it felt like people would hunt me down at the store to ask me the same thing. These are men that you love and respect. People you've listened to talks from in Sacrament meeting. People you've heard bear their souls during priesthood meeting. People you've listened to all your life share testimony every first Sunday of the month. Constantly hounding you to do something you're not even sure benefits, as global society as a whole seems to mock missionaries more than anything else.
So, I ask again. Did you really go because you wanted to share the joy in your heart? Or did you go because "that was the social expectation"?
I didn't go because I don't need anyone to tell me I'm doing the right thing. I only need to feel like I'm doing the right thing.