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.
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.