The sad truth of the matter is that in the video/computer game industry if you are going to get any shelf space in the major retailers, you really have to be 3D - otherwise the purchase managers who decide what stores stock and where they will be displayed will not stock the game. why should they stock even a couple copies of a new 2D game designed for a few niche grognards when that space can be given to Civ4 which is at least keeping up with the technology.
Especially in the USA, where the shelf space for PC games is already reduced to the back corner of the store, with maybe 10% of all shelfspace in the store vs the flood of console titles, unless the developers keep up with the direction of the industry (which is towards 3D) they will simply be relegated to complete "niche" status and not stocked at all - and players will be told if they want the game, order it online from a "specialty dealer who handles niche titles", since very often stores will decide not only to not stock titles in brick & mortar, but also not list 'low volume" titles if they have an online retail presence, since they would likely not have stocks on hand to fill such orders.
In the end, going to 3D is a question of economic reality. Stay with 2D, and be seen in the industry as uninventive, conservative (nay, reactionary), and struggle to find new customers who have become so used to the adoption of the new 3D technology they can't imagine playing anything so "old school" in look and feel.
Paradox, like any business, has to continually grow its customer base if it is going to be able to move forward in terms of the development of new titles. And given the direction game development is going today, that means 3D has to be incorporated. It comes down to capitalism 101, if you fail to respond to changes in market preferences, your growth slows, then atrophies as the original customer base moves on with no new customers to take their place.
For those of us who have been loyal fans for years, that can be a tough pill to swallow, especially if we do not like the direction that the industry itself is evolving towards. but in the end, while Paradox I'm sure values us as customers, over time our numbers will decrease and if they do not replace us with new customers who have different expectations of what a computer game needs to contain to be considered "contemporary" then you end up as a business owner stuck in a "niche" with a stagnant customer base slowly shrinking over time.