Any lens that extends when zooming or focusing is going to require air to enter and leave the lens. I am not aware of any lenses that completely seals requiring a pressure different between the inside and outsize of the lens. The 100-400 may be more susceptible to dust since a larger volume of air is exchanged when zooming (which is why it is designed as a push-pull in the first place) and there may be a more direct route for dust to get in since rotational zoom mechanisms may trap dust, depending on how they are constructed.