Once ETH goes Proof of Stake fully, ETC will be INFINITELY more profitable to mine than ETH will be.
For that to happen, price of ETC must go 20-30 times up
The price won't matter, once ETH goes fully Proof of Stake income from mining it disappears as there will be NO MINING any more on it.
That's also going to dump profitability on ALL GPU mineable coins across the board though as the (currently) 9 million or more GPUs mining on ETH look for new homes, and will hammer profitability even harder on any remaining ethash coins as the E3 units in use by then look for new homes.
But thats exactly why ETC price will have to rise to make it profitable
Or it will go unprofitable for a while and shed a bunch of hashrate - a far more likely option IMO for pretty much all GPU-mineable coins when The Next Big Shakeout happens.
Much like what happened to X11 coins when folks GPU mining Litecoin dumped out of it due to the Gridseed GC3355 + coin price dump drove GPU miners out of Litecoin in early 2014.
It would certainly do some bouncing up and down until it would settle at some (profitable) level...but this "asic" is different from others
because its not order of magnitude faster or more profitable, driving GPUs out...it certainly makes no sense to buy new GPUs, but
for already working rigs, it doesn't change much