Easy Street:
Am I right in thinking that the most commonly-quoted reasons for retention of pilots, despite advances in automation, are that only a human can think flexibly enough to react to any unforeseen circumstance, and that only a human can continue to fly the aircraft when critical elements of automation fail?
Yes you are right in thinking this and on many, many occasions this fact has been demonstrated. Its just that you never read about them because the outcome is that the aircraft lands at an airport (maybe not the intended one) and everyone goes home.
What is being said about the absence of the G/S is that it removed a safety barrier. Put it another way, if you consider the (fictitious) predicate:
An accident is likely if the crew are not that skilled in visual approaches AND are new on type AND there are CRM issues AND the PM is distracted AND a steep descent is required to get on the glidepath AND vertical guidance is not available.
If any condition is removed the accident becomes significantly less likely.
Note: this is a gross simplification, just to illustrate a point