Here's why POS won't work. Its based on the most fundamental premise of greed i.e. you won't harm the value of what you hold. Unfortunately, that same premise will see the downfall of Ethereum. Why? one simple word - Bitcoin.
Bitcoin values are already at a all-time high and owners want to see it higher. The only way to to create demand and inflate values is to either hold the world to ransom (wannacry) or boost its status as a safe haven digital currency. Based on the latter, it may be in the interest of bitcoin holders to see Ethereum fail because there will be a massive selling of Ether for bitcoin.
With POS, the door becomes open for the Bitcoin whales to manipulate a massive failure of the Ethereum network to their benefit. How so?
Bitcoin is worth about 10x more than Ether at the moment. If the Bitcoin whales can accumulate 51% of Ether. They can engineer a failure of the ethereum network. Ethereum holders in their panic will flock to exchange their Ether for bitcoin driving up bitcoin and increasing the value of their existing holdings. As long as increase in value of bitcoin holdings > decrease in value of ethereum, it would be in their interest to do so. Furthermore, Bitcoin wants to be the pre-emininent digital token. To get rid of their closest adversary would surely be in their long term best interest.
So, in a nutshell, the POS basis of using greed to protect itself will in turn cause Ethereum to fail because they forget about the greed of others. Just my 0.0001 eth 's worth.
Each unit of Bitcoin is worth about 7x of ether (at the time of writing). And that's based on price alone, the market cap of Ether is about 80% of bitcoin (which is what we really need to look at).
Simply put, if someone wishes to buy 1% of ethers, he would need to trade in about 0.8% of the entire bitcoin market. Attempting to purchase such a large amount of ethers (51% of the whole market, which is 46 million ethers) will most certainly cause the bitcoin market to spiral down and ether market to skyrocket. If you fail to get 51% to crash Ethereum, the PoS system will then be able to wipe out all your deposit and 18 billion dollars (causing the remaining ethers to rise 2x in value since supply is halved).