Higher speed actually translates into less throughput. Cars take up more than just their physical space; there is also a gap that is needed to stop safely in time--and this gap grows according to the square of velocity.

(You can argue that people don't usually leave sufficient space to stop, but I will point out that self-driving cars, which prompted this discussion, are going to be following those basic safety rules)

No, higher speed always results in higher throughput given the same density.

Your complaint is that cars lower density as speed increases, which is true, but you're still looking at this too narrowly. Consider the bus.