It's easy to double your money on each attack. Send to an exchange, reverse transaction, withdraw. Or, send to exchange, trade for bitcoin, reverse the transaction.
Edit: By the way, that last paragraph in your post is only true for currencies with significant hash rates.
Pulled it from the bitcoin wiki. But I see your point about the exchanges. It's a catch 22 then, in order to create innovation you need to start with a new coin but you have people who will 51% attack the coin to suppress it only to protect their bitcoin and litecoin investments or out of greed to get more bitcoins and litecoins. So older but flawed networks are protected by their size.
I see a few things coming out of this.
protection networks. developer can rent the services of miners to maintain a certain hash rate on the network to reduce the chance of a 51% attack.
merge mining. coins banding together to consolidate hashing power to reduce 51% attack risk.
improved trust. actively developed coins gaining more support and lift their price per coin.