I have had a couple of down bags washed, including washing a North Face bag purchased in 1977 twice. That bag is in regular use with a homeless person now.
Since you are mentioning a 30 year old bag, I believe I can speak to that experience. I think new bags are treating their down with chemicals that perhaps give them different properties. Maybe even properties that allow them to be washed with less degradation, but I doubt it. My experience is the first time you wash the down bag it loses at least 10% of its warmth. It seemed the second time I washed the North Face bag it didn't get much worse.
I also have worked in management in an outdoor company at Lake Tahoe. We sent our bags to Sierra designs to have them washed. I recall discussing it with the folks at Sierra designs and they mentioned you would lose loft and some performance after you wash the bag the first time. However for some bags they were so dirty and full of salt that the difference was negligible.
I guess the final answer is, how dirty do you think your bag is? Is it compromising the performance right now? Why do you want to wash the bag? Do you have a use for it if its performance characteristics are somewhat diminished? Do you have a personal connection to the bag or is it time for a new piece of equipment?
I can understand wanting a piece of equipment to last a lifetime. It is rare though with heavy use. Sometimes you just have to break down and support consumer society. Buy well.