In the near future, I wonder if there will be time limited disposable blockchains, eg some corp protects it's short term disposable blockchain with significant hash, but credit on that chain, is only good for 3 months, much like a prepaid phone card. This may keep the blockchain very small and fast, then that's it.
So only good a a few hundred dollars for local spending. Also the chain would have a limited maximum value, and only be good for local geographical areas, eg local supermarkets, shops etc, so even if you did attack it you would have to travel really far to use the coins, which cuts down the number of possible attacker (though presumably they could be arbitraged back)
Of course the 51% may be the big issue, but if there are lot of theses chains, and I mean thousands/10 thousands, it may be hard to attack them all. Their is some risk you money will disappear, but this is small and offset by speed. Also the corp may under write the value of that chain in case it gets attacked.
This way they can keep out of VISA etc and its really fast.
Just a thought.
Who would run miners on these blockchains? What incentive would they have to do so?
The issuing company, eg a bank, vodaphone, coca cola, state transit/metro authority. As a way to trade BTC/LTC into local chains, eg, you know you need to spend 100$ in food / rail over the next three months, they lock in the price at the time you trade in for their services, that is thier service do not change in thier coin price over each 3 months rolling period. Good for rail services/metro areas. Phones, Coca Cola.