I agree with you about the net benefit in purely economic terms, but it's easy to overweight the benefit when you were born after the costs were paid. Industrialization of production also inevitably led to the industrialization of warfare and we know how that turned out.

I can't say with any certainty that we're definitely better off than if WW2 had never happened. Suppose it had not, and that there had been slower technological development without the necessity of war, so that in 2017 the cutting edge of technology was 2400 bps modems and 300 dpi laser printers (ie a ~25 year technological 'peace handicap'). Is the actual progress we've made over that period worth the ~50 million lives lost in WW2?

Put another way, would you be willing to kill 100 million people now, today, in order to take a 25 year technological shortcut (with no certainty about how well it would pan out)?