All car axles are vented and most have 100000 mile fluid change intervals. Why aren't they worried about condensation?
I am going on vacation, so I won't see TBN for at least 5 days. Trout fishing, camping, and kayaking.

So, I won't be able to follow up...
I think you make a good point. I know most gearcases are vented, and I was surprised to see that front axle was not vented. When mine was leaking, I thought seriously about venting it.
The auto manufacturer put the vent in the rear end by design. The tractor manufacturer left it out by design. Venting the axle is a redesign. I am not a better axle engineer than the guy who designed it. It would have cost them very little to put a vent in, had they wanted one there.
When I tore into it at Christmas, what I saw inside made me glad I didn't vent it. There is a bearing is at the very bottom of the vertical part of the axle that would be the likely recipient of any collected water. It is right near the drain plug.
If anyone redesigns the venting system, it might be advisable to redesign the oil change interval as well to make sure that bearing does not fill with water.
It is a lot of work to repair the axle seals, but there is a feeling of satisfaction too, particularly when you get all the oil out that has little bits of metal in it. It's a bit like lapping compound mixed with the oil. You will never get that out of the hub housing just with an oil change. You would never get all the water out of that section either. But, then again, you won't have any water there if the axle is not vented.
I changed the hydro oil at Christmas as well. I took the fill cap off the other day, and when I looked at the underside of the cap, there were water droplets on it. It's a bit alarming how quickly water gets into vented cases.
New oil, 4 months old, and the tractor was used at least 15 hours in that time, and at least 6 hours box-blading two days before.