I invite you to design such a lens and, if it works well, is reasonably small and light and affordable, I'll be first in line to buy it!
I own two "sealed" L optics, e.g., 17-40 4L and 24-105 4L, and I can feel a blast of air out the rear when quickly zoomed or focused. The 24-105 is said to have a dust shield where the nested barrel slides in and out of the main barrel. A assume a filter-like skirt to help keep dust and moisture out but allow free movement of the barrel and some air passage (hence water and dust resistant rather than water and dust proof). I imagine a 100% sealed system would need more torque in the AF motor than a ventilated one and a larger barrel with air space around elements to allow quick displacement of air when zoomed or focused.
Hmm.
The 70-200 f/2.8L IS apparently doesn't displace any air at all. I tried four tests to get a hint of moving air while zooming in and out very quickly, as well as changing focus very quickly, but got no indication of air moving in or out of the lens:
1) Lens next to the ear
2) Fingers with hair near the rear end of the lens
3) Wet fingers
4) Post-it note
I thought I felt something once or twice with the wet fingers, but keeping absolutely still also occasionally produced the same feeling, so I could only ascribe it to evaporation.
I repeated the experiment with my 17-40 f/4L IS, and I got the same results.
Perhaps my testing methodology is flawed, but there certainly is no
blast of air with the 17-40. My guess is that you've got a bad lens there.