I agree with you somewhat about today being a dark age where people are dumbed down educationally. I saw it far too often in my professional career where I had university graduates who literally could not rub three words together to form a proper sentence - or even correctly spell all three words.
I have a hard time agreeing that (depending on the definition) the European "Dark Age" was prosperous. The Roman Empire, in the west, had completely collapsed by the 5th Century. Europe, after over 1,000 years of having a system of government was cast adrift. Life was not easy and it surely was no utopia (unless you take the literal Greek meaning of utopia, "no place"). Without the Roman army to keep the peace Europe broke up into warring and virtually lawless "kingdoms" where each king was trying to reunify Europe in their own image. Monty Python may have had it correct.
As far as modern technology being based on 19th and early 20th century ideas goes, it isn't a sign of stagnation. Each of the technologies you referenced is based on one or more of the four fundamental forces of nature - gravitation, electricity & magnetism, the strong and weak forces. That isn't going to change, ever, unless there is some other force of nature unknown to us today. Of the four forces, only E&M lends itself to easy manipulation. Gravity is used in modern technology but only in the sense that it is a constant and unidirectional. Other than introducing a large mass we have no way to actually manipulate it. That leaves the weak and strong nuclear forces. Because they are associated with nuclear fusion and fission (strong) and nucleon decay (weak) they play their parts in modern technology but not in common everyday technology that the average person might understand.