I agree that in-game conditions change and therefore Camp - CS rates and spawning rates have to change. I do not agree that they have to cease altogether. 'Barbarians' in the game are a mechanic to compensate for combatively abysmal AI Civs: if it weren't for Barbarians, there would be little real competition from the AI Civs or anything but Natural Disasters for most of the game. Removing Barbarians from the equation by the beginning of the 3rd Era (which has been my experience so far with the new Mode) begs the question of what is supposed to replace them for the last 3/4 of the game?
So, IF the current implementation is as intended, what is supposed to come next?
More Free Cities? That already happens to some extent, as Civs expand closer to each other, there is more pressure to 'flip' cities. But, so far at least, Free Cities never progress to anything - they don't become City States, they don't form new Civs, they simply become part of another Civ, or flip back to their original Civ (the most common occurence in the early game, in my experience). And their military units act mostly like City State units - they don't go invading or messing with their neighbors much, just stay close to home and counterattack anybody coming near. That makes them a very passive replacement for the 'true' Barbarians.
And note that logically, IMHO, Free Cities should have received some attention in the Barbarian Mode. Why don't they ever become City States? Why wouldn't they become aggressive with their military units - especially against the Evil Tyranny that (presumably) was their parent Civ?
And, in a potential solution to the Disappearing Barbarian problem noted above, why couldn't Free Cities become Urban Barbarian Camps? A particularly poorly placed city, with several factions from various Civs all (presumably) vying for power within the city, could/should I would think, devolve politically into such internal dissention that the city collapses into anarchy. That phenomena is not limited to a particular historical era, since there are several 'states' today that have, effectively, become pockets of political and military anarchy, presenting problems for their neighbors that mimic the in-game problems presented by nearby Barbarian Camps early in the game.
More Rebels? This would be a sort of historical solution, in that some of the largest and most expensive wars for 'Real' Civs from the Renaissance on have been internal Civil Wars and Rebellions: look at the Tai Ping in China in the 19th century, or the American or Russian Civil Wars for horrific examples. I have not played a game with Barbarian Clans long enough to get to 'rebel' territory, so cannot comment on whether an increase in rebel activity takes place with the new Mode. Further, I have seen so little rebel activity in any game so far that unless there is a major change, such activity is insignificant: I have seen Civs disappear from Loyalty pressure flipping every one of their cities, but never accompanied by conquest of any of their cities by the new Free Cities, which would be a closer approximation of the Civ-Rending Civil Wars like the ones I mentioned above.
So, if the Barbarian Mode is working the way it was intended, I submit that it is Unfinished, and needs some more work to be fully integrated with the entire game from turn 1 to turn 500 and to incorporate and modify some of the other mechanics in the game already, like Free Cities and Loyalty Flipping as it relates to Civs and Free Cities.