There's a difficulty bomb in place for Ethereum which means the difficulty keep increasing regardless of miners:
Difficulty adjustment scheme
A lot of you have been wondering how we would implement a switch from PoW to PoS in time for Serenity. This will be handled by the newly introduced difficulty adjustment scheme, which elegantly guarantees a hard-fork point in the next 16 months.
It works as follow: starting from block 200,000 the difficulty will undergo an exponential increase which will only become noticeable in about a year. At that point (just around the release of the Serenity milestone), well see a significant increase in difficulty which will start pushing the block resolution time upwards.
So, a year on, the network will continue to be useful for roughly 3-4 months, but eventually will reach an Ice Age of sorts: the difficulty will simply be too high for anyone to find a block. This will allow us to introduce PoS, perhaps via Casper, if it proves itself.
Nobody GPU mines Litecoin anymore. It's ASIC territory, just like Bitcoin.
So no replacement of Ethereum once it becomes extremely hard on GPU mining? What about Dash?
I'm planning to purchase GPU of $200-$300 and then buy more time-to-time. But it'd be pretty bad to see once ether mining becomes useless. GPU miners will have to explore some sort of replacement.