51% attack is non-profitable for attacker => probability is low
It is profitable, dead coins still exist and if that address ever becomes active again they will no longer be considered dead. But the amount of dead coins there are contributes to a function that will lower the difficulty by a small percentage(once a threshold is reached).
Edit:
-Or do you mean the decrease in value of the currency would make the attack pointless?
Yes.
Basically if you will create a device which will be hashing at >=51% of of overall BTC hashrate it is much more profitable to just mine new blocks or receive transaction fees. Assuming that you actually pay for your hardware.
Ofc if attacker is just very very rich and wants just to destroy BTC.. then attack is not pointless ; )