If you actually read the (whole) post, their concerns are quite valid.
I am not against the civ changing with the era, but do recognise that the message that is being sent by this mechanic is that these cultures no longer exist. Part of the story of history the game is trying to tell is that empires rise and fall and no culture remains the same for a thousand years, let alone six thousand.
The problem is, cultures do not completely disappear either. A European nation who had a vast continent-spanning empire during the game’s Exploration Age will still exist today as an independent nation. Of course the global empire of trade and conquest is more appealing from a game design perspective, but does that culture cease to exist in the Modern Era, obviously not!
This is even more problematic when depicting the colonised people themselves. The Maya and the Shawnee and the Songhai still exist. It is natural that the game depicts them at their historical height, but the game has them inevitably collapse and disappear in later eras.
Firaxis have to be careful here. I don’t think it is likely they will allow you to continue into a later era with the same civ, and even if you did it would be at a huge disadvantage. Of course Humankind had the same exact issue, even if it let you “transcend” as a culture this clearly wasn’t playing the game as it was designed.
I think much of how this goes down will depend on the variety of options of civs to pick in later eras, and whether there is a tangible and visible legacy of who you played before.