You'd still have to compress the nitrogen going one way to zoom, and pull the nitrogen against air pressure going the other way. Focus motors powerful enough to do that, plus including movable airtight seals to keep the nitrogen inside, would make the lens a serious chunk bigger, heavier and more expensive.

The dust that gets inside from breathing while zooming rarely affects image quality to any meaningful degree. Excessive humidity can lead to fungus growing on the glass though, which really can ruin your contrast. It's normally good enough to keep your lenses in a box with some silica desiccant during summer to prevent that.