Marketers are smarter than you give them credit. "Dumbing down" games doesn't make money. Playing to a wider audience makes money. And a wider audience doesn't mean dumber audience, despite what some elitist intellectuals will tell you.
A wider audience means looking at Civ's closest cousins. What games are similar to Civilization? You've seen them thrown out here. Age of Empires, Age of Wonders, Age of Something or Other. Rome Total War. Call to Power. Tropico. They'll look at the sales figures of all of those and figure out which of those audiences Civ is poised to "steal from". Then they'll ask questions like what can be added to Civ to grow that audience.
They won't dictate to Firaxis what features will or will not go in, but they could make general comments like "people seem to go nuts for more customization" or "we need to get Civ online right off the base release". They may even say "people like games that let them relive history, as opposed to rewriting it" -- or the exact opposite "people enjoy the scenarios, but they seem to love being able to rewrite history and ending up in a situation where the roles are reversed".
Marketers, if they're going to have any say in the product, it's giving very broad and non-specific goals. That's if they have any say in the product at all. With a game like Civ, they probably trust that it's done something right so far, too.