Gordon,
Fail-safe aspects; agreed - MCAS operation, where output involves trim.
The trim runaway scenario is part of a systems safety assessment applicable to all models of 737, this has to consider failures such as a short circuit, etc, anywhere in the trim electrical system.
The ‘master’ cutout switches inhibit the power supply, but still depend on pilot detection of errant trim movement.
This situation could involve the autopilot, assumed in some circumstances to automatically disengage, or with manual electric trim, or just at random. Each alternative, and the overall situational context, workload, speed, … would affect the crew’s ability to sufficiently understand the situation as a trim runaway.
Yes, only half a story, but if trim runaway is to be considered then how might an improvement be found.