When the govt pays money to someone, it doesn't (directly) reduce productivity or social utility. It just moves money from one hand to another (or one person's hand to another's). So if govt pays a soldier 50k, it costs zero in social utility.
But when govt forces a person to stop productive activity and instead be a soldier, net social utility is decreased by whatever he would have produced. So if it's a 50k producer, it is lowered by 50k; if a 100k producer, it is lowered by 100k.
You're premise is that a soldier in the army only contributes to social utility if he is paid for it. Yet the production is the same regardless of the payment.
If 101 people live in a country with average salaries of $50,000 and government taxes 100 people $500 each in order to fund a $50k salary for the 1 soldier, net social utility (including the soldier) is $5,000,000. If the government doesn't tax anyone, but instead drafts the 1 soldier without a salary, net social utility is $5,050,000.
If you don't count the soldiers labor towards social utility, then it would 4,950,000 and 5,000,000 respectively.
What you seem to be saying is that anything that generates a salary is considered social utility here, yet in truth anything that should earn a salary is economic production. According to your theory the solution would be to give the guy a salary and then tax it away from him. Then we'd definitely be ahead with the draft.