One facet of the evolution of commercial transonic transport design since 1970 years is that L/D ratios (alternatively, the M * L/D) have increased only mildly over time. For example, see the following photo. I always found this fact to be incredibly surprising, because over the same time period the capabilities of CFD techniques to optimize airfoils and wings has increased dramatically. I expected L/D ratios to be much higher today than they were in the late 1960s.
The reason I've seen given which reconciles the simultaneous lack of significant advance in L/D ratios (a measure of aerodynamic efficiency) and immense increase in CFD capabilities (which should increase aerodynamic efficiency) is that designers often "trade" improvements in aerodynamic efficiency for reductions in structural weight. Firstly, I'd ask, is this a correct explanation for the only modest improvement in L/D ratios seen over the past 25+ years?
Note: one example that's given of this is that a reduction wave drag achieved by reducing shockwave generation in transonic flow can allow aircraft to fly faster, or fly the same speed at greater wing thickness, and the latter is often chosen to reduce structural weight.
Secondly, this question got me thinking, if the above explanation is correct, are there any references that attempt to model what the "counterfactual" L/D frontier would be for transonic transports if aerodynamic efficiency was not traded for weight improvements? That is, how high could we reasonably push modern L/D ratios for commercial transports if that was our main concern?