Indeed, I can see how many infrequent flyers wouldn’t notice gradual descent or lack thereof, but it’s just not plausible that experienced flight attendants (the only kind working on planes this large) wouldn’t be aware of it if they were focusing on their jobs the way they’re supposed to be.
They have probably made that trip dozens of times, maybe even more. They have a routine from take off to landing. They know when they have to be finished collecting trays and things from the cabin. When they finished that and still flew on without hearing anything from the cockpit I would have thought that would be a hint that all was not well!