Let me ask. Suppose the civ series went all the way to iteration number 100. Do you think that every single fan - even the most hardcore ones - would all agree that every game got better every step of the way?
In the favourite version of civ poll, you'll see that not everyone who isn't voting for civ5 is necessarily voting for civ4. There are quite a number of people who liked one of the earlier games more than they liked civ4. Does that mean civ4 was not better than the previous games? Who gets to decide whether a game is better or not?
To answer your last question: Each man for himself, it's a subjective matter of personal opinion.
To answer your first question: I would assume that the vast majority of the fans, with the exception of irrational, as determined by commonly accepted basic logic, people, would expect each and every iteration to be an improvement over the last.
You and Jharii were talking about expectations. You also said people can have different expectations. I have yet to see an industry or market where an expectation of non improvement is acceptable ... or even considered logical.
The new iPhone comes out, some people would expect better camera, some people would expect better reception, some people would prefer different design and some people would prefer different accessories. 1)They all have different expectations, 2) Each will have his own opinion of the end product 3) They will all base part of this opinion on a comparison with the most recent version of the previous iPhone.
People can have the same expectations, for instance the expectation of an improved Civ V over past iterations and still disagree on their opinion of the end result.
There are of course aspects of the game that can be argued more objectively, like wether the game is streamlined or not, wether mechanics work as intented or not, wether the AI is capable or not, etc etc. One's opinion can't change facts, it can only ignore them or accept them.