You have, though, made it harder for the economics to work in favour of apartment buildings and public transit, and as a result made life more difficult for people who choose not to, are unable to, or can't afford to drive. Because people in the suburbs will vote in favour of thigns that make their motoring experience more pleasant, they will demand new construction have off-street parking (even of the residents don't drive), making units tens of thousands of dollars more expensive and less pleasant to walk near, and free or dirt cheap parking in city centres. They will force transit stations to have places for cars to sleep, not people (aka park and ride). Meanwhile, those people who don't drive still bear the negative externalities of the extra vmt and their taxes pay to enable it for others.
In addition to the economics, you've made the public realm more hostile to walking and cycling; car-friendly places are human-hostile.
More vehicle-miles traveled corresponds more emphysema, more extreme weather events, more noise, more light pollution, more cancer, and just more dead people in general. It's impossible to say who or when or how (except in the case of crashes) but every substantial road expansion kills some of the people nearby.
Well, maybe those 50% achieve some benefit to themselves and society once they arrive at their destination. Keep in mind, if those people happen to be arriving at jobs they had previously, that's not a greater benefit than before.
However, the movement of people down the road itself is a social detriment. Most people don't to spend time in traffic for kicks and the cost of fuel consumption, pollution, wear on highways and wear on cars is significant. I mean, if somehow I could spend only five minutes in my car per day and still get to my destinations, the benefit would clearly be far greater.
Only if you know how the occupants in these additional vehicles were transiting previously. If they were previously carpooling, or if they moved away from a good (but slightly less time efficient) public transport option, then the social benefit would be somewhere between minimal and nil.
You’re leaving out the taxpayers subsidising their commute.