https://www.independent.co.uk/news/science/it-is-faster-to-s...
> Standing-only escalators had a "throughput" of up to 151 passengers per minute, while an escalator where commuters were still allowed to walk saw around 115.
Your thought experiment's completely right, but in real world circumstances, once an escalator is above a certain height, people rarely use the walking lane:
> But a 2002 study of escalator capacity on the Underground found that on machines such as those at Holborn, with a vertical height of 24 metres, only 40% would even contemplate it [walking up]. By encouraging their preference, TfL effectively halves the capacity of the escalator in question, and creates significantly more crowding below, slowing everyone down.
(from https://www.theguardian.com/uk-news/2016/jan/16/the-tube-at-... , which has a great graphic showing the two systems)