Certainly not all potential alternatives would be worse, but I think you'd be hard pressed to find a privately developed road system anywhere in the world that really matches the publicly built systems.
Unfortunately that point is irrelevant. The only thing that is relevant is looking at the total cost to society for the Interstate system (including opportunity costs) and the total benefit to society for the Interstate system... and only then comparing it to alternatives.
The problem with Interstate fans is that they have zero imagination. They are unable to consider how our history would be different without the Interstate system. They can only take our current state and subtract the Interstates and consider how we would be worse off. Of course we would be worse off! For over 50 years, we have organized our cities, our way of business, our cultures, our markets, our car purchases, and our entire lives based on the fact that you can get from point A to point B in a car in a particular amount of time at a very low (marginal) cost.
We all laugh about the flying car idea, but one reason it's not here is that there's no need for it. We have the Interstate system and it's cheap (per mile) to drive on it (don't confuse marginal cost per mile with overall cost). But with anyone with the slightest bit of imagination and ability to think outside the box, it is pretty clear that without the Interstate system connecting the cities:
a) cities themselves would be more compact, a la Europe... bye, bye suburbs and long, environmentally-expensive commutes
b) air travel would be a much, much bigger market, and it would be mass produced similar to cars
You people are looking at the chicken and the egg and wondering how much it would suck if we didn't have the egg! It's a nonsensical comparison because you're missing a very obvious causal link.