I really dont think GPU mining will die anytime soon due to there being so many new coins launched that require proof of work.
The mining also add's a layer of security onto the coins framework.
Yes some of the bigger coins like ethereum will move onto proof of stake, but it's not in the bigger coins that you make your money on. It's the smaller coins that will boom is where you make your money.
As the GPU hardware get much stronger and more powerful, all the more reason to have proof of work with mining.
As of now, the current hardware out now is more than capable of solving blocks really quickly. Ethereum has slowed down the rewards per miner as they do not want the coins to be distributed that quick to the market.