I really don't understand what you're trying to say. The NCR and Caesar were founded on societal institutions of Old (ancient) World concepts. The past isn't dead and forgotten. If that were true in anyway the various factions would not have modeled themselves after the old world symbols.
The King's gang has no idea who Elvis is, they only know him by the name King. For all they know it was a place of worship so they look at Elvis as some kind of old world deity. They're not parodying Elvis, they're literally worshipping his image.
The statement that Bethesda is ignoring the "central concept" of Fallout is out in the left field. You cite New Vegas dlc (I'm going to assume Lonesome Road) as harboring this concept, yet the base game contradicts that because the old world is still alive and kicking: NCR, Legion, Kings, Khans, Chairmen, White Glove Society, Omertas.
NCR = Old world California government
Legion = Roman civilization
Khans = Genghis Khan's golden horde
Kings = Elvis worship
Chairmen = Greasers
Omerta = Italian Mafioso
White Glove Society = Posh, elitist consumptionists
Like, what is this REAL Fallout you're talking about? Even the Enclave in the original games and the current iteration believes itself to be an extension of the United States Government. Most of the organization draws descendents from the original political cabinet before [censored] hit the fan. They literally thought that once the bombs were dropped, the top-tier officials would come out swinging and destroy communism and reunite the lands. This screams old world clinginess. So, please, can you elaborate on what it is you're trying to say? Because all I'm seeing is deep-seated resentment for Bethesda piloting the franchise.
Edit: Adding to my post, I feel that Awesomepossum sums it up really well.