"Never" Why? The next version will have even higher poly count animated graphics ("Zoom in and see your Rifleman picking his nose!" "Watch the ancient Romans queue up at the public latrine!") while still making the game harder but not deeper as you move up in difficulty. Simply increasing the advantages given the AI as you move up just makes it necessary to do more micromanaging and to memorize more minutia and at some point that just becomes tedious. It doesn't make sense to me that the tech tree is that same at Settler as it is at Deity. For one thing, it bewilders new players and for another it leaves you with nothing to look forward to. The same goes for victory conditions. Leaving the current ones in place at lower levels is okay but, it would be nice to have others (Spiritual? Ecology?) become available, along with the new techs for them, as you move up. The same should apply to Wonders for the same reasons.
If Civ 5 ever appears it will tout the number of new buildings, the 'enhanced' graphics, new units, new Civs, bigger maps, etc., while I (Having at long last opted out of the CPU, memory, graphics card arms race) will probably never play it.