The charts above do not indicate the nature of the longitudinal stability issue - seen by the pilot as change of stick force with speed.
The MAX problem was twofold.
First identify the nature of the stability problem, as required for certification, which might be fixed with a relatively easy ‘bent tin’ mod.
Second was to make the MAX handling (stick force vs speed) as close to that of previous variants so that there was minimum type difference training - no additional simulator time.
The optimisation task was further complicated by the -800 certification being marginal for these aspects with respect to the latest certification standards, avoided by ‘grandfather rights.’
So where a simple ‘bent tin’ mod could be certificated, new, variable - ‘auto reconfiguring tin’ with speed would be required for type similarity. As I recall there was previous discussion on this re tests of LE ‘flaps’, strakes, etc (#13 and previous threads)