'… why didn't Boeing just change the parameters of Elevator Feel Computer (Elev Feel Shift Module) as it was already available on 737NG?'
I don't know, nor have access to Boeing's thoughts at the time.
There may be clues in the references to the integrated MCAS within the exsisting STS (NG). In terms of speed, adapting the STS would be logical, and when the 'Manoeuvring' requirement was identified - need for AoA and Mach, then the existing 'digital' computation from DADC (air data and AoA input) and FCS / FCC with stab trim output, was more practical for test and development than modifying the 'heavy engineering' at the back of the aircraft.
Last edited by safetypee; 5th Aug 2020 at 16:19.