I think one variable is that people wash their quilts more often these days than they did back before every home had a washing machine/dryer.
The less often you wash a quilt, the longer it will last.
Personally, I don't routinely wash my quilts...only when they show signs of being dirty. That said...since it's just DH & I and we are both clean people, my quilts don't get washed very often.
I learned this from my aunt who quilted all her life and died at the age of 92. She said to only wash the quilts when necessary.