Degrade to dust probably works better in theory than it does in practice.
So many players refuse to bother with t90 because it degrades to dust. They don't bother calculating the cost/hr, the simple fact that it degrades to dust is a large enough turn off that they'd rather use any other armor instead.
I don't know why they can't seem to balance anything properly either. Nex is still probably the... third? hardest boss still (Vorago, Araxxor - We could debate where RotS and Raids fall, but I dont think it interferes with the point) and does she really deserve to be so easily solo'd? It just seems it leads to excess supply of items in the game with nowhere for them to go. Looking at the price of seismics still, it should tell you that if they could make the boss fights decently lengthy, and decently difficult, the prices of items CAN retain a decent value. T90 didn't NEED to degrade to dust, it could of been fairly rare from some fairly difficult encounters, but I guess they wanted to try a different mechanic.