The style of the older Civ games never actually seemed more "Mature" or "realistic" to me, it looked blocky and pixelated (Civ1) or dull and undetailed (Civ3).
It seems like that whole Zelda: Windwaker thing ("Zelda, moar liek Celda! Amirite?"), only this time it's more like "Civilization, moar liek cartoonilization! LAWL! Sid Meier? Moar liek KID Meier! LOL!". Really, only teenagers insecure about their own maturity would be bothered by the graphics. If you need to play dark and edgy M-rated games to feel like a man then chances are you aren't one.
That's it. I just can't believe the attitude of the OP actually exists. It feels like I've seen this reaction with so many things before, it's tedious. I don't think Civ will become more or less cartoonish, they'll just keep making some Civ games that vary in their flavour and texture. I don't think we're going in one or direction or the other. CivRev is CivRev, and no one can really say anything about it yet. Already whining about the downfall of the whole series because of a "spinoff" game that isn't really in the series to start with, I dunno.
Now sorry, I am an adult and appreciate complex and mature games like Civ, I will go away and write a dissertation about the downfall of my favourite game while I enjoy a cup of Pinot Noir.