Per Western Mountaineering, American manufacturers of high quality down sleeping bags,
Down also is known to have the greatest longevity, and it will outlast any other insulation by 3 to 5 times, making it the most economical choice after its initial investment.
Having worked in specialty outdoor retail, I would describe your situation as unusual but not rare (if that makes sense). If properly cared for, as you have described, high quality down will last for numerous sleeping & compression cycles. When a sleeping bag is being stored uncompressed, there is very little degradation that occurs to the insulation. Instead, it is the repeated compression cycles of being stuffed and unstuffed, as well as oils and dirt, that degrade the insulation.
With regular weekend use, a down sleeping bag can often last 10-15 years, especially for higher fill power down. In comparison, a synthetic bag might begin to feel colder in as little as 3-4 years. In professional use (e.g. a mountaineering guide), where the bag is being slept in every night and frequently stuffed and unstuffed, a down sleeping bag will begin to feel colder somewhere around 300 nights. All of this, of course, depends on the particulars of the situation. Also consider that a sleeping bag generally encounters low amounts of wear and tear when compared to almost any other piece of gear--it will spend its time outdoors almost always either safely inside a backpack or inside a tent / bivy sack.
As a parting thought, the difficulty for manufacturers to source high quality down has actually increased over recent decades--fewer people are eating geese. However, manufacturing tolerances, sorting & grading methods, and shell fabric quality have all increased over the same time period.
