Is Proof of Work is going to lose popularity compared to Proof of Stake? Ethereum is going to PoS. 
You don't need to care that! Ethereum is just an Altcoin in the cryptocurrency world, we still have many cryptocurrencies using the PoW algorithm, included Bitcoin.Is there going to be a lot of hash (computational) power idle because of shifting to PoS? 
Where that power be used then? Will it address such decentralized computing projects as Golem, Storj, Sia, etc?
Maybe many miners will sell their GPU to get back their investment. Instead of worrying about that, why you don't be happy you can buy cheap GPU from them? Besides that, ETC, XMR, or ZEC are the best alternates crypto of Ethereum for GPU mining.Where that power be used then? Will it address such decentralized computing projects as Golem, Storj, Sia, etc?