A great example of how important gracefully failing is. Broken escalators are just stairs, and broken smart-locks should just be locks. If that isn’t the case, then they aren’t actually smart.
Apparently when an escalator is broken the space between steps is uneven, and the escalator is metal and sharp, causing safety issues (although I've written other theories below in further comments).
People are more likely to fall climbing a stopped escalator than stairs due to the different stair height and depth.
But I don't live in London.
They are sometimes blocked off but it’s unusual.
In fact I found then quite reliable compare to my old college's elevators that saw less than half the use.