The definition of a declining society is the loss of the ability to uptake new ideas. It has been the watermark for decline in every recorded civilization that we know of.
A new idea emerged that could have solved all the problems of that society ... and they failed to make any use of it. This is such a simple indicator with such profound implications. It implies amongst other things that the inhabitants of a declining civilization are theatrical and contrived. They seem to move around, be alive and have responses ... but this is an illusion. The lights may be on but nobody is home. All sorts of incredible innovations whizz around them and they act as though they are reading prescripted dialogue and acting out automated behaviours like the animatronics at Disneyworld that play the Founding Fathers. They don't take any notice of anything happening around them. The eyes are dead and the souls have vacated the premises. All that remains is the closing act to bring down the show. Change or adaptation has become an impossibility. This is the way all human civilizations come to an end.