All I've worked out is that the kind of arterial 30 limit road that you're talking about, with pedestrians well separated from the road by verges, good visibilty, not many traffic lights etc, is not going to be a target for a 20mph limit.
So all of this discussion is completely pointless. You're talking about conditions completely different to those meriting a 20mph limit. You are using your own very specific example of roads not warranting a 20mph limit to try and put a case against 20mph limits in general.
Anywhere with rows of shops, kids playing next to the road, schools, difficult visibility, and other factors that make it not a dead straight roll-along would however be a good place to consider a 20mph limit. The stop-start nature of those roads plus the major safety benefits would defeat the slightly reduced mpg arguments.