Because the breech from the lake water will continue unabated after the pumps shut down shortly. Why would breeches at the levys in Plaquemines bode ill for New Orleans? That's the better question.
Doesn't it stand to reason if levees are having trouble downriver that the ones in NO are having just as much trouble? Not to mention all that water coming down from rainfall north of NO.
How long can the earthen levees stand? I don't think we are getting the whole story on the Mississippi River levees but that is just my opinion.