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?
This is just a matter of taste, what I see here is not to turn off POW POS, although POS does have some advantages, I think like this, there will be no POS if there is no POW, there is a POS because updating is not yet in POW. And I am sure, the POS here is not to turn off POW, because the blockchain is too dear if it is turned off because of its longevity.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?