Two issues: one moral, the other practical.
1) The government does not own you. When warfare was less advanced and a nation's military strength was largely dependent on the volume of trained soldiers it had, you might have been able to get away with arguing that national service improved national security. These days we'd be bringing it back just to instill a bit of discipline.and turn citizens into more rounded human beings. Flying in the face of liberty, this would be central government trying to impose an arbitrary set of values on people against their will - robbing them of, what, two years of their life?
2) Would it really turn out a generation of disciplined, law-abiding young people? I'm certain the opposite would be the case. You can't erase decades of modern values and give people a mindset that hasn't existed for 50 years. People wouldn't shrug and gamely go along with it - there'd be uproar. It's be the Poll Tax Riots multiplied by 1000.
Even if you did drag people into it, you'd create swathes of disaffected young people who are deeply deeply resentful of the state for years to come. Thousands of young people would choose to flout the law on principle, even if it meant prison, so it would effectively criminalise a massive percentage of young people for very very little gain.
It's an absolute non-starter. Popular dissent, political suicide, huge cost, imaginary benefits and an affont to our freedom all rolled into one.