The too-fast one in Paris was 12km/h, not 4km/h which would be OK.

https://en.wikipedia.org/wiki/Moving_walkway#Trottoir_roulan...

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.