Genghis,
"these chaps were experienced and current pilots unlikely to make such an elementary error. "
Only one of them made the mistake, the other two did not know what he was doing. PF used up 27'000 feet and PNF then had a go with less than 10'000 feet left. You are there to monitor the other guy but cannot see what he is doing, so you assume he is getting it right, at least to start with? In my book that gives you a single point of failure and that is a bit naughty.