if you had a pilot directed to manipulate the thrust levers to maintain Vref plus 5 and a proper descent rate, and he or she DID NOT MAINTAIN SPEED, at least we would hope the OTHER PILOT would notice and do something about it (other than crash).
so too the automation...direct it to hold speed and if it doesn't do so, DO SOMETHING ABOUT IT other than let it crash.
There are some days you just have to be a pilot. On that day, the redundancy of having two (in reality three) pilots in the cockpit/flightdeck/whatever failed. Really incredible failure points, considering that level of the profession.