How is the decision to introduce movement assistance (moving walks, etc.) in terminal plans made? What factors are involved?

This depends upon the circumstances of each terminal.  A traditional rule of thumb that many terminal planners use is that a walking distance of 1,000 ft. plus or minus is a theoretical maximum walking distance goal for passengers once inside the building.  This distance takes the average passenger about five (5) minutes to traverse.  As aircraft wingspans and the number of gates increase at a terminal it becomes more challenging to meet this theoretical goal.   Additionally the perception of convenience, or inconvenience, is equally important.  For instance, it is not always necessary to introduce a moving walk or other conveyance, such as an automated people mover (APM), after a passenger has walked 1,000 ft. depending upon the circumstances of the physical plan and other variables.  Bridges, connectors, and other static environments that are visually perceived by passengers as long distances provide excellent opportunities for improving convenience by providing assisted movement.