@lrbaumard - If you think they didn't do some form of research or focus testing, you are wrong. But let's expand your view: the expansion model paradox games use is, in some ways, riskier than putting out sequel. The point of an expansion is it's quicker and cheaper than starting over again, both in terms of development and audience. That means they have sell it to the audience they already have, someone who owns the game already. The more DLC a game has, the less attractive it will be to new players. This is why EA creates bundles for their Sims content: they try to put together an enticing entry point that takes that edge off.
So sure, perhaps you think this doesn't cater to you and maybe you are right, but it caters to other players. The success of industries and park life has apparently shown this is a popular format and given the developers a reason to keep using it. In the end there is will reach a point where the DLC is no longer profitable enough to justify support, and it's around that time we'll see an announcement for Cities 2.