The notion that a single planet has to be largely self-sufficient as an independent unit is a bit of prejudice that comes from our upbringing on Earth. On Earth, in terrestrial economic terms, it's generally relatively expensive to get things off the planet and back onto its surface in one piece. For this reason, we think of planet Earth as a practical (if not absolute) boundary for economic transactions and resources. In the larger context of a solar system spanning society, it's costly in absolute terms by current-day Earth standards, but in the larger context, it's also relatively cheap to ship things down into the Venusian atmosphere from Mercury, Venus-crossing asteroids, and even Mars. In fact, one can make it materially inexpensive by applying lots of energy, which shouldn't be nearly as expensive by that point.

It's probably true that Earth's gravity well will limit trade of material goods between itself and the rest of the solar system. But there's no reason that a solar system spanning culture couldn't exist as a largely separate entity. I suspect people will live out there just for the idea of it, and that nation-states will help in the endeavor just to propagate their own cultures.