This whole RA thing obviously offends so many electronic design types who are posting.
The serious failure case is covered by Boeing - you cannot engage both autopilots for a Cat 3 approch and autoland if the RA system is not operating as it should. Any fault in the RA system at any other time is, to use Rainboe's word, trivial. Please understand that. There is nothing wrong with the system.
There are countless, possible trivial failures that can inconvenience an approach. Crew deal with them every day - it is part of the job. Maybe the windshield wiper packs up in heavy rain, maybe (God forbid) the autopilot drops out on intercepting the glideslope and you actually have to fly it yourself. In this particular case, disconnecting the autothrottle would have done it. It was, after all, telling them with a RETARD caption and by not applying power to maintain airspeed. What more warning do you need? The autopilot would have stayed connected. There was no technical problem. The problem was the crew not monitoring the approach - full stop.
Hopefully we shall learn why they did not monitor the approach but to criticise the RA system is not helpful.