It is very good UX how escalators slow down when nobodies on them but keep moving.
It makes it clear the direction of movement and how the user has to position themselves so they can ride it without thinking about it- but it saves power from slowing down
I'm actually a retired electrical engineer, used to do aircraft avionics. Anyway it's a simple thing to measure amperage loading on the motor to set a threshold for sleep and wake. But in engineering there's always more than one way to accomplish a task. An optical sensor and timer would be another way to do it.
If you were using a vfd in order to control the speed of the escalator, a lot of vfds have intelligence built in, so you could just wire it up and have the vfd take care of everything. On the other hand, I can see a bunch of reasons why a current might work really well for a short term demo and start to fail immediately after. Start to get things gummed up or also trying to deal with very small people riding the escalator, the trigger point might be difficult to keep straight.