Chomsky had an explanation for why most of the world unquestioningly believes the USSR was the true socialist/communist expression - propagandists from both the worlds superpowers wanted it that way for the better part of century. Socialism/communism appealed to post-recession Americans, so the political right pointed to the USSR dictatorship as the inevitable result, and on the other side Stalin and his successors were rolling in socialist rhetoric (being a philosophy widely regarded at the time to be morally superior) to cover the stink of dictatorship.
In the end terms mean what people want them to mean, and that changes over time. My brief reading of Engels
Change occurs because someone with power wishes it so. Unfortunately the world is almost as likely to get a Vista or Win8 rather than an XP or Win7.
Regarding the "millennia of decline" : Middle Ages, once called the Dark Ages... but apparently some good things happened in there too besides the societal collapse and depopulation, so the unreservedly dark term has become less popular. It was an unambiguous step backwards though that lasted a rather long time.
"Religion is something left over from the infancy of our intelligence, it will fade away as we adopt reason and science as our guidelines." -- Bertrand Russell