A 51% attack only rolls back a chain for so long as one sustains the 51% hash rate.  It is a good trick to do a few temporary double spends.  If you want to kill a chain permanently, you have to sustain the 51% hash rate.  That's quite an investment in vandalism, and moreover, you will probably be recognized (IP address launching all the false blocks).

I really doubt that anyone in his right mind is going to do so.  It is not happening with crypto that have the same PoW function and wildly different hash rates, so why should this happen to ETC ?