Shirley there must be an indicator that shows if the plane is pressurizing as it is supposed to. Once you go past 10K feet, the onset of hypoxia becomes more and more of a threat. Well before the plane had reached cruise altitude oxygen masks should have deployed.
Don't call me Shirley.
I was working an adjacent sector when Stewart's plane went NORDO (quit talking). He was at an altitude where sudden decompression would certainly get your attention, but still you would have time to descend to breathable altitude.
That they did not descend indicates hypoxia slowly set in, probably from malfunctioning equipment. Investigation showed that pressurization switch was on auto, so pilot error was improbable.
The sector continued trying to contact him to no avail...often not a big deal, just a radio problem. But when he continued climbing through his assigned altitude it was suddenly a VERY big deal, fighter was borrowed from a warning area and vectored in but unable to contact him.