If depressurization incapacitated the crew, wouldn't the plane just keep on flying at 38,000 feet?
The crew apparently had the wherewithal to set the jet for a controlled descent which indicates that the crew had oxygen at least initially.
My leading theory as well - if true the FDR will indicate the cabin pressure as well as the warning (usually triggers between 10-14K ft)
another report indicates not a single passenger made any cell phone calls during that last 8 minute descent- to me that suggests everyone was unconscious- at least in the passenger cabin. If the pilots were aware enough of something wrong to adjust settings, it had to be fast- at 38K feet you lose consciousness very very fast and without any defense against it or even awareness it’s happening