I noticed in Berlin, escalators without people often run very slowly. They speed up as soon as someone steps on.
In Copenhagen, they stop. This is presumably more efficient, but it's easier to assume the escalator is broken, and a bit disorientating.
I suspect there's no reasonable way to retrofit them to existing ones - probably requires a different type of electric motor and AC speed controller plus sensors which is getting to be a pretty big job (so expensive even if the parts aren't).
It takes TFL about 9 months to replace out an old escalator for a new one. I think I'm correct to say they have 50 year operational lives.
Seriously? Why? Genuinely curious
It takes them a few days to replace the ones in local supermarkets, including the flat conveyor ones to carry the trolleys.
If I were to hazard a guess this is because of the soft clay that London is built on - on the one hand the soft material makes it very easy to dig tunnels, on the other hand if those tunnels are dug too shallow they will very easily destabilise the foundations of the buildings above. So you have to to build them at a depth which would be prohibitively expensive somewhere built on harder rock.
Personally I prefer to manually climb the stairs and I’m often surprised by how much space there is between escalators (it often seems like 50% of the width of a tunnel is just metal closing up the space between the parts of the escalators you can stand on) but I think my opinion is rare and it would be inefficient to have a flight of stairs that nearly nobody uses.