civ has never been a mainstream game. it has been a "mainstream" in the turnbased category. with other gametypeplayers taking part in the gameplay. but still it has been a turnbased strategyworld, not for the big mainstream market.
and good that you took sim city 4 in your post, because that was a big f*ckup.
as i said 3d is nice, but the turnbased strategymarket isnt a market for 3d. first priority is the ai and gameplay NOT 3d. offcourse 3d and visual effects are nice, but turnbased strategyplayers are alot more demanding then other gamecategory players. in other types of gamecategories you often want something nice to watch,etc. while in turnbased its basicly all about good algoritms(ai,etc),maps,etc. which all takes ALOT of resources in itself.
where 3d is no must at all, but its nice. but its like buying a superluxery car just because it looks good. and then after buying it you realize that you cant handle the loan, because it just costed to much. so you would have to sell out your house etc.
3d is no high requirement in strategy games, gameplay is!. 3d is a bonus(as long as it doesnt cost your "house"(gameplay) to buy that fancy "car"(3d)).
we do agree that civ4 has reached the mainstream market long time ago, through it beeing a very good turnbased strategygame of its time. but the gamestyle is still not that mainstream. adding 3d makes it more mainstream yes. but then the question comes down to if its going to focus on beeing a mainstream game or a turnbased strategygame. (turnbased games do require alot of resources, but thats usually in memory and cpu, without even 3d. add 3d and ....well).
and as you said, it will sell good anyway. but that doesnt justify everything. it has become a brandname loooong time ago, the civ series however havent involved much.(actually the civgames seems to just get more and more incomplete for each release. and thats sad. one should think that when they earn so many millions that they would care about making a game more or less complete).