In a little bit over 2 years (2 years, 4 months) vanilla CIV IV sold just over 3 million copies according to Take-Two. In just under 1.5 years, CIV V has sold around 1 million copies according to VGchartz, (not the most accurate measurement - but most likely in the ballpark).
IMO, people continue to attack the game because they felt they were mislead as to what the game would be like when they originally purchased it. And yes there was a demo they could've tried beforehand - but this was Civ - and they thought they knew what they were getting.
"The thing is, Civ V is a big sloppy kiss/love letter to our fan community. We want it to be for the hardcore. We want to make it as accessible as possible, but Civ Rev kind of took care of that route. Its for the people who want the kind of Civ-lite [experience]. Civ V is for hardcore PC." Dennis Shirk
http://www.vg247.com/2010/07/09/interview-civilization-vs-dennis-shirk/
This one contradictory comment, combined with the supposedly streamlined gameplay of CIV V, alienated
a lot of hardcore fans, and you can see this if you visit any of the major Civ sites, (WePlayCiv, Apolyton, Realms Beyond, CivFanatics - and virtually all foreign Civ sites), during the week or two period when CIV V was released. Consequently, the CIV V sections of these forums are now all but dead, (CivFanatics being the exception) - although CivFanatics traffic is nowhere near what it once was. Many of these disgruntled players still post on the forums in hopes that Firaxis will do a 180 of sorts on any future expansions or versions of the series - and in a way they've succeeded.
Religion is coming back, despite the devs stating that it didn't coincide with their vision of diplomacy in CIV V upon release. On that front, Diplomacy itself was made less of a black box, in that we now know why a Civ likes/dislikes us. This too went against the initial stated design goals regarding diplomacy and how it was supposed to be "mysterious." Without all of the negative posts about CIV V that bemoaned it's lack of features and overt streamlining, we might've been getting a very different first expansion.
And yes, CIV V is still loved by many in the community, and it's highly played on Steam, and not all hardcore players hate it - but it's alienated a greater number of players than any Civ before it. (And yes, CIV IV and III also alienated players of earlier versions, but not to this extent, and not for this long). The question is, has CIV V brought in enough new players to replace those that have now left the series?