I think cryptonight ASIC miner will increase 50x difficulty for every cryptonight, so consider whatever you are mining now going to 50x expensive to mine for GPU.
But yes it may decrease coin value, because mining cost per coin for ASIC will be so each.
Need to wait and see how it helps newer coins
Is there no way to modify the algorithm such that ASICS for a particular modified version become obsolete?
I think Monero is tweaking its mining algo so that ASIC don't become profitable, so yes there are ways, but its mixed approach, some coins are changing algo and some are not.
So things will become clear in few weeks. But i think the overall impact of ASIC has been good for coins, like bitcoin, litecoin, dash. So hopefully it will be good for cryptonote coins as well.
Not sure if bytecoin is changing because cryptonote coins are either monero fork or bytecoin fork.