If we were designing the interstate system today, that's actually a fun thought experiment. What would we change? I would probably focus more on throughput -- cars can be half as wide and still seat tons of people, 5 lane highways today could easy be 8 lanes of narrower cars.
As you point out, several studies have shown that over time, travel time converges to the same value, despite additional capacity.
However, I think it is important to note that more lanes means more people are still getting from point A to B.
Peak hours are always going to cause congestion.
The only problem is we can't afford to build enough lanes. There is a limit to how wide you can go, and up or down is vastly more expensive than at grade. We could do it if we wanted to spend the money - but most such places find that a better bus and (not or!) train network is a better investment.
Why would you go narrower? That makes problems worse for trucks and other cargo conveyance; note that in general, newer highway standards tends to increase lane width, not decrease them compared to older standards.
(I guess you can see the same effect with railroads--the UK railroads have smaller loading gauges than newer networks like the US, Germany, or Sweden).
In fact, the cost of building & maintaining the highway system is quite astronomic.