I guess it all depends on how they're cared for. Food stuff/plant products tend to rot if they get wet and even damp fabric starts fallin apart after awhile if they ain't dried proper.
Anyone that has a canvas tent knows (or learns) not to pack it wet,, mold/mildew happens pretty quick.
I don't know why but I have a big bag of used up/dirty walnut media, I know a lot of CF guy's use it in those Caldwell shooting bags and I know you can pack pretty tight.
I like to keep "sand bags" a little loose so they form better. Like BB I settle my hand into a bag and hold my rifle with ML shooting and my CF hunting rifles because it's more like my field and off hand shooting "hold". For CF target bench shooting "bagging" is a different ball game than I'm talking here and I don't bench shoot ML.
I did learn that if I bag the gun and dial in the sights, I'll have a different point of impact when I use a field/offhand hold. That's why I support my hand and hold the gun.
When I made those 60 for the gun club I made another dozen for myself, it was a week long project, cutting, folding, sewing, invert, spray paint a logo,, even paid a 10yr old neighbor kid $5 an hour to just hold the bags while I filled'm, he earned $20-