They're only sustainable if the demand for unrestricted fares in Y and C holds up.
That's applying rational economic theory, of course, but as soon as you have irrationality (a carrier under Chapter 11 protection with a vital interest in cash-flow and bugger the debt levels, any state-owned carrier, any state-subsidised airline) then they set the market price and you'll have to have pretty good USP to compete at higher fare levels.