deadheader,
Many aspects of your question were considered in prior, mostly closed threads, but never pursued to a meaningful conclusion.
Reasons for this are difficult to establish; thus specific answers remain unknown because they would depend on what tests Boeing conducted over the many years of 737 development. The results of these tests should have been available to the FAA as submissions or supporting data in certification.
Starting with the original 737, it could be expected that a full range of evaluations were conducted and used to justify safety, procedures, and training. But it may be difficult to establish how much of this data has been transferred with grandfather rights to newer variants, not always repeating the tests.
Spot checks within the normal operating envelope would support a new series certification, so too data from mandatory extreme structural and handling flights VD, MDF.
However, how far the original assumptions have been stretched for abnormal operation might be less clear, e.g. recovery from trim runaway. See the related thread
Evolution of 737 Runaway Trim Procedure
Changes in the emergency drill suggest adaptions to modifications to the trim system / electrical system; but not necessarily to any aerodynamic changes with aircraft growth.
Thus a key question is to what extent was the runaway trim procedure validated in new variants. Critical assumptions involve pilots’ recognition of the failure and reaction time in isolating the electrical trim, but amended drills appear to lengthen this period. If so, then the likelihood that the extent of the tail plane deviation will increase with failure, with matching difficulty in maintaining control and establishing a safe trimmed condition using manual trim wheel / elevator. Note changes involving grasping the trim wheel, together with a reduced size of the trim wheel.
EASA posed certification questions for the NG, which were satisfactorily addressed by manual trim in situations where electrical trim was unavailable - either by design (minimising effect of runaway), by motor characteristics, or crew effort; still unknown ?
The effect of the reduced size of trim wheel in recovery was not published, thus assumed to be negligible; however, with accumulated assumption, extent of trim runaway, reaction time, this could now be questioned - see FAA review. Anyway it might be difficult to validate or justify this situation based on flight test - particularly if the test was unsuccessful !
It is probable that the response to EASA only considered normal trim operation (that was the question), and not that of extreme tail plane deviation nor further adverse effect of elevator input for recovery.
There are no simple answers; if any at all.
We cannot be certain about the effect of a failure; there is no ‘certainty’ in certification. Judgements are made about systems’ design and evaluation, technical, human performance, reliability, etc. This is a forecast based on what is known (assumed) about the systems probability of failure and the likely outcome, the latter being most difficult to judge particularly if relying on human input. Whilst the failure rate might be the same, the outcome perhaps less certain based on recent events.
How far back this issue applies - previous variants. This, as above is unknown (unknowable), but growing concerns from pilots and operators, and more recently regulators might reflect a serious safety issue.