What struck me at the time was that there wasn’t an immediate trigger for action in the way the fault presented itself. The 737 trim is active all the time during flight; in fact it is unusual for the trim wheels to *not* be in motion for any length of time. STS, MCAS, config changes, CofG changes, etc.
The Boeing checklist trigger for trim runaway at the time was “continuous uncommanded trim motion”, which guards against an electromechanical runaway, but that wasn’t what happened - it was a software failure that only moved the trim under certain circumstances. How could you tell the difference between, say, STS doing its job and and an MCAS failure? The answer is, in the short term you couldn't, and abnormal operation appeared the same as normal operation unless you had a long diagnosis period, by which time it was too late.