No, please explain it to me.
Ok first of all, taking every idea from a game that is already out doesn't make much sense. You either want to play Simcity 4 or you don't. If you want to play simcity 4, then go play simcity 4. These are 2 different games.
Also, a small team has to prioritize what features get in based on their time frame. Which features are most important toward the game they want. What features can be put in later down the road. What features can be modded in by others. A team can't do everything and thus you have to prioritize, otherwise you get a big mess of a game that no one wants.
You have to realize time is a major factor. It's a business.
Why didn't Bethesda look toward the mods the community created for Oblivion and implement them into Skyrim? The reason is because, that isn't them. They make the game they want to make. The base and the modding tools. Those who want specific features, they leave that up to the modding community. Why should they go out of there way to try and please every single individual? Isn't easier to just create the base and then add onto it from there and let others as well?
Keeping in mind, development would be very different when it comes to games without modding capabilities, because then you will be trying to add in more features. Which requires a bigger team and a more time. This does mean though once the game is released, if you don't like it, than that is it. There are no mods, and there is no community to add the things that you want.
Yes or no. Is Cities Skylines. . not a good base? Personally I think it's far beyond a good base to add onto, and I don't see how anyone could think other wise.