What is this? It's harder to design? So hard that doing one good 3D map will be as hard as making a dozen awesome 2D maps? Please. I'm not buying that for a second.
Well why not? Have you designed one yourself?
It doesn't add anything? Yes, actually, it does. I just got done with a SotS2 game on a map with a center area and SIX spokes coming in at 90 degree angles with an empire starting at the end of each one. Doing that in 2D means you just get four coming in or you get spokes coming close to each other instead of just meeting in the middle. What 3D add is simple - it adds more space.
It's too bad the one and only game you bothered to mention was a flop. Why it was a flop will be irrelevant to most. As in, it doesn't matter if it was because of the 3D maps or not. Don't pretend that adding more space is always a good thing.
To reply to both quotes, there is a reason why most games don't bother with 3D, would you agree? Whether it's because it's harder to design, or because it truly doesn't add anything, or whatever, I can't say. Though what I can say is this, only do 3D if the game you made has the mechanics to make use of and compliment such a feature. It is my understanding you would do this for any feature in the game. Assuming you want your product to sell anyway.
2D and 3D are completely different beast. Stick with what you know yes? Creativity and ambition often result in failure, it's why you don't see much of it. Games are too expensive to develop nowadays. For most companies failure is not an option. Wouldn't want to lose your beloved stardock for a silly 3D map, would you?
Obviously for the sake of effect I over dramatize a bit.