And this means games only exist as an economical niche.
Before I am going to comment on your statement, I would like to give this quote:
But I know this: if I play a computer game, I don't want Sims, I don't want an MMO and I don't want any form of mediocrity. I'm paying for the game exactly not to be a mediocre waste of time. If the developers are not happy with the profit they are making from the Civilization players worldwide, maybe they should start looking for another job. I'm pretty sure they were not starving all these years.
The point is, a TBS like Civilization always has been some kind of niche product.
In its niche, it was very successful.
Giving up that niche means leaving the environment in which you have been successful, trying to explore surrounding for which you may not have been equipped.
People say that the biggest and strongest living carnivore would be the icebear. So, only knowing this we would expect him to even dominate jungles and Serengeti, no?
Yet, it doesn't.
Let me give another example.
Porsche (the car manufacturer) is well-known for their sportscars. High-priced, quite exclusive sportscars. One could even say they dominate that market to a certain degree. Almost anybody only slightly informed about cars knows what a 911 stands for.
Now, nevertheless, during the past 10 or 12 years they have issued other cars, too.
Boxster, Cayenne, Panamera.
All these cars are sportive, too. They have to be, since they are made by Porsche. People just wouldn't accept a middle-class sedan from Porsche.
What ever they do, they have to create sportive cars. And they do and have done so.
Nevertheless, the new types were not called "911-casuall" (Boxster), "911-SUV" (Cayenne) or "911-limousine" (Panamera).
Because they went into different niches with these cares, niches in which a 911 doesn't fit.
They had to get different names as otherwise people would have had expectations which could not be fulfilled.
Coming back to Civlization.
Civilization was a niche product, played by a special kind of customer. That doesn't mean that these customers would be any better or worse than the customers for RTS, FPS and what not more.
But, clearly they are different. They have different intentions about which kind of game to play, how to play it and what to expect from their game.
Civilization to a certain degree was a label like "911".
Trying to move this same label (more) into the mass market may be successful in the short term, as people are thrilled to get "their" Civilization (911) now, which previously was out of question.
At the same time, long-time customers are shocked.
To appeal the mass market consumers, things have to be stripped.
Civilization has been reduced in terms of functionality, choices and options. The equivalent would be to have a car called "911" but with less powerful engine, no sportscar interior, weaker brakes and worse road handling...
For the mass market, it may still appear as a very sportive car, but for the long-time Porsche driver the new car has lost what made its fascination.
And after some time, the new customer may detect that his new car is not so good in transporting things for a whole family. Suspension may be uncomfortable (even if not as hard as in the previous 911s), seats aren't really made for people with more than 200 lbs and so on.
The result may be that when releasing the next generation, you won't have any of both customers anymore.
Your traditional customer won't forgive that you've changed the 911 for the mass market, the new acquired customer may have already changed the car for a pickup.
Civ5 has genes of a Civilization game, but isn't it anymore.
They should have given it a different name: "Sid Meier's Civ: Exploring New Frontiers" or whatever.
The undisputed weaknesses would still be around, but it wouldn't have to compete against a game in which class it no longer plays.
Firaxis/2K wanted to make a quick buck out of the brand name. Seems that they succeeded in this.
Personally, I doubt that this can be repeated. I doubt that Civ6 (if it were ever to come) will be a big success.
Old fans are lost, new fans most probably will not be that faithful. After all, Civ5 is for the mass market. You rarely find much dedication amongst mass market customers.