I'm not an artist, but I think the issue is much more nuanced than some of the posts here make it out to be.
First off, 2D vs. 3D is just a medium of expression, and neither is going to inherently yield a more visually appealing game. Bold outlines or character that may be traditionally prominent in 2D can be added to 3D, but it requires a broader mix of technical and artistic abilities. You might have to write shader code for example.
Cost wise, it's also very complicated. 2D doesn't necessarily mean raster or pixel art. If a cartoon look is appropriate, you can use a pure vector approach that allows you to use inverse kinematics or other techniques that reduce animation cost. If the cartoon look is not desired, you can then add shading or textures later to vector-based outlines. I'm not sure if there is any way to automate this based on some lighting parameters yet, but it seems like a technically feasible approach that could reduce the amount of had drawn frames required. At this point, the problem would become somewhere between traditional 2D and polygon-based 3D, since you'd need heightmaps or something to light and shade properly. If you are hand-drawing each frame, then yes, 2D does get expensive, especially when taking different viewing angles and rework into account.
3D on the other hand, can require a fairly substantial investment into modeling, texturing, and rigging. At larger companies, there are specialists that perform each of these tasks. Once this is done, minor rework is not as big of a deal compared to hand-drawn 2D frames. Major rework is still pretty expensive, as sometimes you need to basically toss the model and start over, or the work requires multiple people to be involved.
Also, in both 2D and 3D, the cost really depends on what you need to do. For example, while it adds to the playability of an RTS, does a turn-based strategy really need its 2D assets to face different directions? How detailed do your animations and textures need to be for 3D if you're never going to zoom way in?