I'm not so sure ETH will crash. When BTC took a dive of 33% and took a lot of coins with it, ETH was one of the first to recover. I do believe ETH can/will correct to what it's "really worth" if it keeps pumping too fast, too soon, but at the same time, it's telling that each selloff sets a higher value floor for the coin.
Personally I planned for long-term investing with a target of $30-$40 ETH token price AFTER PoS saw implementation. It was never supposed to go this high before PoS even showed up. So hey no real skin off my nose if things get a little ropey.
A few other points I'd like to make that are probably relevant to the current discussion:
Proof of Stake will apparently only be used 1% (actually .099%) of the time:
http://www.coindesk.com/ethereums-big-switch-the-new-roadmap-to-proof-of-stake/
So mining will continue for awhile yet.
I also want people to consider the ramifications of an actual attempt by BTC holders to dump crypto for fiat.
Even the major exchanges like Kraken are beginning to experience liquidity problems. People looking to cash out inflated ETH (for example) for fiat may run into trouble. GDAX is apparently still reliable, but a few people are starting to experience trouble at Kraken and (to a lesser extent) Coinbase.
Combine this fact with the high transaction fees of BTC, the currently high failure rate of BTC transactions, and the long transaction resolution times currently plaguing the BTC blockchain, and you have a recipe for . . . actually, stagnation.
In order for BTC prices to actually drop due to a run on fiat, someone is going to have to agree to sell BTC for fiat at prices below the prevailing price. This behavior will have to continue for awhile, covering a large number of transactions, to meaningfully reduce the price of BTC. If you go to an exchange that has large quantities of BTC available for purchase (either in the exchange's own pool, or by linking sellers up with buyers who are willing to hold their BTC "on the exchange" rather than a private wallet), there's no problem. But selling? If the exchanges are experiencing low liquidity in fiat and there aren't a lot of buyers lined up to get your "discount" BTC, then how can you even complete a sale without agreeing to a massive price reduction at the time of sale?
As things stand, it would be much easier for BTC HODLers to convert their BTC to some other crypto than to try for fiat.
A big running of the bears for BTC might scare some of the new kids out of the market, but it's also going to drive a lot of market cap into other crypto coins. The only bear run on BTC that can hurt the other coins is if there's a push to cash out for fiat, which is technically becoming too difficult outside of one-off, brokered transcations.