Grit is what actually destroys a rug
Wool hides soil extraordinarily well, which is the trap. A hand-knotted rug can look acceptable while carrying several pounds of fine grit deep in the foundation. That grit is angular and abrasive, and every footstep grinds it against the wool at the base of the knot where the fibre is thinnest. Fibre cut this way never recovers. The rugs in the worst structural condition are almost never the oldest ones; they are the ones that went thirty years between washes.