The only tangible argument, in my mind, against adding further Ages to the game is the issue of DLC bottleneck.
For simplicity's sake, let's say they add ten new civilizations to Antiquity, Exploration, and Modern before the new Fourth Age DLC is announced. That DLC includes ten new "Information Age" civilizations. We then would have 20 choices for Antiquity, 21 in Exploration (including Shawnee), 20 for Modern, and 10 for the Fourth Age. Naturally, that would feel imbalanced and strange to players who are used to greater player choice.
Because an Expansion Pack being a one-and-done DLC delivery, no further civilizations would be added to the additional age. Unless, of course, they walk the dreaded path of DLC for DLC. If Firaxis/2K is wise, they will avoid this misstep at all costs. That said, it would be the only way out of the bottleneck, unless the Fourth Age does not require a civilization switch.