If say the guaranteed protection against forks for the short term is up to a year, that means clients need to sync with the network at least every year. If you're always syncing, you won't be fooled by sham blockchains because it doesn't follow from your last trusted blockchain tip. If it does follow from your last trusted blockchain tip and you were syncing at least every year, then it's a short-range fork, and the consequences will be severe for the attacker -- it's unlikely to happen.
Even without syncing often, the client can get the blockchain hash from external trusted sources (or many trusted sources from existing validators), and sync thereon. Both of these solutions can be utilized to create a practical and secure solution.
You are outsourcing the trust problem, not solving it.
Vitalik says cryptocurrency developers should "reach out to LessWrong more", in order to not be ignorant of advanced computer science and game theory topics.
https://blog.ethereum.org/2014/12/31/silos/
I think Vitalik raises several major important points, namely:
* Bitcoin's $600 million wasted on electricity yearly in order to secure the network is a huge problem. Various Proof of Stake algorithms are attempting to improve upon this. The ideal solution would be one which combines the low cost of the Proof of Stake algorithm, with the high security or Proof of Work. I not enough of an expert to be able to say which solution is best, but both the Ripple consensus algorithm and the Bitshares Delegated Proof of State algorithm look promising. I am interested to see what algorithm Vitalik will choose for Ethereum.
* ASIC miners are a problem due to the resulting centralization. They have also lead to increased centralization in Bitcoin, as the network is now increasingly controlled by a few large mining pools.
* Blockchain technologies have a great potential to change the world, and solve governmental and organizational problems that society is facing. Beyond simply revolutionizing money, blockchain technologies could also be used in the future to support prediction markets, voting/consensus building, exchanges, etc.