"...I figure that washing it regularly in plain water can keep some of the oils out and avoid washing it with down wash, which I'm assuming can degrade the down after many washes... i dunno if this is correct or not. ..."
Sounds good to me. The big sack that comes with most good bags is primarily for dust control. I prefer to hang a bag from a high hanger, though. The baffles will support each bit of weight rather than having the weight of the shell on the down. I don't believe it really matters. But, I was a carpenter 40 years ago...easy to fix a closet.
The water is likely a good solvent for salt and uric acid/urea...from your sweat. These are water soluable and rinse easily. The acid will act like a detergent so oils are removed. Unfortunatly, this also removes oils from the feathers. Soo, even plain water will eventually degrade down. I would guess about 40 years, only a guess, since it depends on each person, how much you sweat, how much you wash it, how well you take care of it, how much you use it.
ANY soap or detergent will strip down of essential oils eventually.