Depends. I don't foresee it being much more desirable to attack Bitcoin, even if the hashrate of the network drops significantly. The profits gained from an attack is far lower than the cost and the complexity of its execution and double spent Bitcoins can potentially just be worthless still.
What if the attack is not due to profit motives? For example, Governments perform an orchestrated attack or some mega rich trillionaire is bored and wants bitcoin gone.