Really this reflects how much "planning" is (a) historical accident and (b) powerless against bigger economic forces. The old high street concept was very dependent on having someone in the house who could go shopping during the weekday. Which, in the present era, basically limits you to pensioners and other benefit recipients. Pedestrianisation would probably help in some of those cases but it's not a magic wand. And of course many of London's "high streets" are completely vital to its sluggish surface transport movement.