I feel like the London sewer case does not fall into the same category as the... problems of the human design. It was designed to be efficient and to meet the needs that could reasonably be foreseen. And then capacity was multiplied for the unforeseen. The human body, meanwhile, was "designed" to be just sufficient. The only time evolution really selects for efficiency, is when two adequate designs are present at the same time and place, under the same conditions.

If I tried to put it into words I feel adequately describe it, it is like when I build an SSTO spaceplane in KSP. I want it to do everything, so I slap all the functions on it, and the first iteration is a mess. I fix the issues that immediately wreck the mission, and the second iteration is still a mess, but now it is only fatal if you screw up. Usually the third iteration is refined and streamlined, with abundant redundance and leeway.(My most recent one lost half its airbrakes(not even symmetrically) during a reentry and I did not even notice until much later.) If we assume that man, and life in general, was designed, the one in charge seems to have stopped at the second iteration in most cases, where things work OK, but there are obvious legacy issues that could be fixed with relative ease.