The thing is that these limits are for inactive people. So yes, technically, there is enough partial pressure of oxygen at 40k at 100%. But pilots dealing with a decompression are going to be on an initial oxygen deficit from the time taken to react to the decompression and get their mask on, and after that will be very stressed and dealing with a great mental workload. So, staying at high cruise levels is still folly.
Emergency descents are often rushed in the sim, causing errors and omissions, which is highly undesirable. Once the mask is on at 100%, the immediate threat is dealt with and the rest of the memory items should be done swiftly but methodically - it should not be rushed. But dawdling and deliberate time wasting is definitely incorrect.