How accurate are mathematically derived "movable middles" (known as "gettables" in The Hendry Model parlance I recall but may be wrong) vs. more in-depth research on positioning ladders/motivating potential for key segments?

I’m glad you brought up Hendry which is a system I have worked with on and off for 40 years.  Hendry’s strength was the modeling of behaviors and did not get distracted by attitudinal measures of brand loyalty or preference.  Hendry would say that the shape of the beta distribution is affected by the degree of loyalty formation and that those in the middle of the curve are more movable and also where a new brand will get more of its triers from.  Our math would be consistent with Hendry in that we both use Beta Distributions.  I am not familiar with the term “gettables”.