As with other shorthand terms designed to eliminate case-by-case by analysis, when “unsustainable” comes to mean merely “bad,” the results can get silly.
We know where the term came from. Demonstrating the genius of central planning by distant bureaucrats, when the Soviets in the 1950s and 1960s decided they had better uses for the river water flowing into the landlocked Sea of Aral (namely, cotton production), and proceeded to divert the rivers into canals where more than half the water was wasted through leakage or evaporation, the results for the Sea of Aral proved “unsustainable,” in the sense that the sea is now more of a saline puddle, while the ships that once sailed her now sit rusting on salt flats, miles from any water.
(From another view, I suppose the Kazakhs or whoever now dominates this area can “sustain” their cotton irrigation indefinitely, so long as the local legal system prevents those who have suffered economic consequences through the destruction of the Aral Sea from collecting full damages. In this way, the situation is similar to the way the Owens River in southeastern California now flows through a big tunnel to thirsty Los Angeles — former Owens Valley farmers may not like it, but this setup is “sustainable” so long as Los Angeles has the political muscle to keep anyone from blowing up their pipe.)