I took it many times, it didn't feel particularly dangerous but you had to pay more attention than on a regular moving walkway. What ultimately killed it was reliability, availability was too low. With more deployed I guess the kinks would have slowly disappeared, but the market just doesn't seem to be there.

Not sure but I think I read a while ago that they were removed due to unreliability (it's a while since I've been there myself).

It was very clever how they did the acceleration/deceleration - the "tiles" of the walkway fit together in such a way that each could slide on top of the next one, and at the two ends the tiles would gradually slide closer together (decelerating) or further apart (accelerating).

That's been tried. Never Stop Railway, 1924.[1] The drive system is a variable pitch screw between the rails. Large screw pitch between stations for fast travel, much tighter pitch in stations for very slow movement along the platform.

Never tried again with that kind of drive, although there are park rides where the loading platform moves. This requires safety devices and staff to prevent people jams at the end of the platform.

[1] https://www.youtube.com/watch?v=EX_MlWL7YKM

And you can only hop on/hop off a cable car at predetermined locations. To keep average speed high, those tend to be spaced relatively far apart.

I’m not sure about the “takes a lot more space”, and I definitely doubt about the “costs a lot more money”. Using outdoor escalators as proxies, I suspect outdoor moving sidewalks will need lots and lots of maintenance. If you want to have some guarantee of service you’ll also need multiple sidewalks side-by-side.