Is Proof of Work is going to lose popularity compared to Proof of Stake? Ethereum is going to PoS. 
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?
1. Yes, it is caused by more and more platforms are looking for the more conservative energy and to change the consensus from POW to the POS is the answer for that. 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?
2. There is no, as so many hashrate power will be distributed into the remaining POW coins in the market. I expect there will always be a lot of coins are still using POW in the future. even with POS movement of ethereum and that will not give a lot of impacts. But those POW coins will be more difficult to be mined as a big hashrate that will be distributed from ethereum to another blockchain that used POW consensus.
3. In my prediction is so many of them will to the cryptonight coin just like XMR and many more.
About your last question and the answer is it will not.