As I have stated I would rather have a 700 down sleeping bag than a 900 down sleeping bag because the 900 is going to collapse under any weight of pressure BECAUSE IT IS SO CAOLAPSEABLE/EXPANDABLE.
Just my $.03 worth
I don't think so. The topic is "Fill-Power
". If you reflect on that terminology for a moment, it simply means that some down has more power to fill a space than other down. That's one reason I can tell a WM bag by feel in a rack of various makes of down bags-- the WM down "pushes back" at you. The other down feels like cottage cheese.
Perhaps think of it like the power of a metal spring: Will a spring of lighter weight with a given loading ability fail sooner than another spring with a lower loading specification and begin to "sag"? Not really. Only if its elastic limit is exceeded.
I interpret a higher fill-power rating to mean that the down is actually more robust rather than weaker. In fact, I would expect down of lower fill-power to "go limp" sooner because the material was more floppy to start with. I don't have enough experience to make a scientific conclusion, but that's consistent with what I have observed.
Just my $??.?? -- whatever you want to value it at. ;-)