The truth is that if Civ 5 doesn't do well in sales, there won't be a Civ 6 regardless of what we say on the forum - so we better hope it does do well in sales if you want to see the series continue.
Like it or not, game publishing a business. It cost a lot of money to develop and publish a game.
I manage a business and a single employee that you pay $60K to a year cost you nearly $120K in expenses after you figure benefits, travel, office space, utilites, office supplies, telephone, computers, etc... If you get a 10% return on that employee, you have to sell $1.2M in revenue just to breakeven. Add that to the fact that it might take three years to develop a game, 10 people working on it (and I have no idea if it takes 10 or 100), then the marketing/sales dollars for it - well, you get the picture. The cost add it quickly.
And if the company doesn't get a good return on investment, it will either go out of business or publish other games. The company simply doesn't have a choice.
And cash flow's a . Even if your profitable, if the money comes in at the wrong time, your still bankrupt. Because you don't have the money to pay your bills. And right now, borrowing money for most companies is still difficult - not impossible, just not as easy as it used to be.
So launching a new product or service is a huge gamble. In the gaming world, it could literally be a bet the company gamble. And the game has to appeal to a large enough audience to make sure it pays all the bills, not just to the hardcore fanatics (which I put myself in that catagory so if it offends anyone, that group includes me).
Its a tough act. Make it interesting and enjoyable for the diehard fanatics but appealing enough for the causal user. I have friends who play Civ 5 and can't get much above Chieftian - they simply don't play enough to understand the game. And I would guess that the majority of players - by that I mean 80% - never play higher than Prince. And like it or not, those are the folks that will fund the next Civ 6 expansion.
And for those who think the DLC are great money makers. I seriously doubt it. The great majority of players will never buy one. And even many of the fanatics will only buy a few. I suspect the profit per unit on a DLC is higher than on the game itself just because it doesn't require any basic research into a game engine - but the actual $ amount and actual revenue generate won't be very much compared to base game sales.
Its really more of service to this community.
If the company was really looking to make more of revenue killing, they would have packaged all the DLC together and released them as an Expansion Kit to have mass appeal. Not to get to a limited target market.