ETH going to POS will NOT make GPU mining obsolete. If you want to get an idea of what it will be like, wait until the next ETH ICE AGE ~30% difficulty bomb ramp up on September 24th. That will make ETH unprofitable or marginally profitable for most people until the ICE AGE is defused and the reward reduced in the planned hard fork for October 9th.
So moving to POS won't be the biggest issue but the ice age update in October could make it less profitable or not at all profitable?
Side note: From my knowledge miners make the currency popular and if they make this ice age update take profit away the currency may be in trouble as less people will use it!