I don't like the demonspawns either, to be perfectly honest.
In a lot of ways though, I don't understand the "liek features but haet FF" because I had a moment or two like that, then I just modded them away.
The singleplayer is supposed to be the most major component of the community (at least that's what's said every time I mention anything involving MP) so there's not an issue with compatibility.
I found changing a few numbers around in the XML to be so EASY that now I almost have a really low tolerance for someone bringing up "X is broken/too expensive/too much" and screaming at the developers. If you really hate a feature in FF it takes a minimum of effort to disable it in most cases. The slow-build-improvements system? Easily fixed. The demonspawns? Not so easy, but ask around and you'll find it's actually in the difficulty files. (Civ4HandicapInfo.xml)
There's a difference between the above complaining and offering constructive criticism, mind you. Thankfully here it seems to be mostly constructive criticism.
There does come a point when changing all this is a lot of work, but as someone who's looking at releasing his own mod(modmodmod, but w/e) into the world, I have to say that the game I want to create will not be the game everyone wants to play. But that doesn't mean I'm going to bend over backwards to support people who want X, Y, or Z added/removed/etc. That would just make the actual problems that concern me harder to track, because I'd have no idea what tiny little iteration someone was playing, and whether I moved a letter or number off in this versions' xml file but not that one.......... it would be a nightmare.
In short, though... FF is already customizable. Depends on how much work you're willing to put in, though, and it is a bit more work than checking off some options in an installer. The end result can be a lot better, though!