"Last time I checked, the ducks and geese stay warm, and they swim around in the water too."
Quite a bit overly simplistic. Many processors of down wash off a good percentage of the natural oils in the down and feathers. There is some left, but nothing like it is when the ducks and geese are wearing it still.
Course even more importantly the ducks and geeses' bodies are constantly replenishing those natural oils which help to keep the down very water resistant.
What they are trying to do with DWR stuff is almost like replace the natural, nature made semi-durable-WR with a more human made hygienic, less smelly, and longer lasting one. Ever smell low grade and not well processed duck down--people report it's not all that pleasant smelling (some have reported that the cheap Walmart down bag has a noticeable smell for example). Ironically though, in some ways it will perform better as far as moisture goes.
However to better apply the DWR, i think they have to strip off more of the natural oils, which *could* possibly shorten the longevity of same. But some claim the opposite, that say fluorocarbon DWR treatment, makes it last longer. I don't what or who to believe on that one.
Suppose we won't fully know until a number of us have had a bag or quilt with DWR down for some 10 or 15 years minimum.