Sure, there exists a point where the input flow is great enough that traffic must move slower. OP's entire argument is based off of this. The problem is that there are many things you can do to slow down the flow rate even further, and merging poorly creates this "turbulence" which wastes further flow.
And, it's clear that no improvement in merging behavior can beat the maximum road occupancy, however we can approach that limit much more closely.
I think that conceptually modelling traffic as fluid flows is quite clever, and the "turbulence" idea is particularly satisfying.
Sure, there exists a point where the input flow is great enough that traffic must move slower. OP's entire argument is based off of this. The problem is that there are many things you can do to slow down the flow rate even further, and merging poorly creates this "turbulence" which wastes further flow.
And, it's clear that no improvement in merging behavior can beat the maximum road occupancy, however we can approach that limit much more closely.
I think that conceptually modelling traffic as fluid flows is quite clever, and the "turbulence" idea is particularly satisfying.