@pamelamarquis - The best way to play just the base game, is to not use any of the futuristic items from the DLC. This would include any ploppables with the blue hexagon background (such as parks and water towers) and anything in the City Specializations that have to do with the Academy or OmegaCo. As long as you don't use anything with the blue hexagon background, you won't get any futurization in any of your cities.
If you really want the Cities of Tomorrow items gone, you will need to contact Customer Service to see if they can help. You can contact EA Help at this link: https://help.ea.com/ Log in at the top right, and choose your country from the drop-down box. Choose "Manage my account" and "Manage account details". This will get you to the appropriate contact options to request that they remove the Deluxe or Complete Edition of SimCity and activate the Base Game. Be aware that when you want to play the full game, you will need to contact them again and ask for the switch to be reversed.
I don't know for certain why they chose to make Cities of Tomorrow an expansion of the base game instead of an add-on, but they did. I suspect it has to do with the integration of the items across all facets of the game. But, as said early, if you don't want the futurization - don't use any of the futurized components and you'll be fine.
