At some point, if you go too high you end up in the Van Allen belts, which if I understand correctly are a really harsh place for spacecraft to be. Generally t

At some point, if you go too high you end up in the Van Allen belts, which if I understand correctly are a really harsh place for spacecraft to be. Generally those orbits are used for parking bulky rubbish that's too much trouble to de-orbit. Geosynchronous orbits are above the worst parts of the radiation belts, so maybe that's a good place for the ISS. Or even higher; it doesn't need to be geosynchronous. The main advantage of a low orbit is that it's easier to get to from Earth, but if you're just parking it long-term as a museum piece that doesn't matter as much. I wouldn't expect Lagrange points to have much of a problem with clutter unless we decide to start parking enormous quantities of stuff there. Objects can spread out and orbit the points; as long as they're all going in about the same direction they should be able to avoid running into each other.