* Reverse transactions that he sends while he's in control Prevent some or all transactions from gaining any confirmations
* Prevent some or all other generators from getting any generations"
Source: https://en.bitcoin.it/wiki/Weaknesses#Attacking_all_users
The first is that it isn't very fair. The reply to this is that the current balance of money isn't fair either, we just didn't get to watch it happen. And as people have said elsewhere, it's not reasonable to expect a currency to solve social inequality.
The second is that, if the cost of mining a bitcoin goes through the roof but there is not enough economy to make money on transaction fees, then people will drop out of mining. However: difficulty is not automatically increasing, it's proportionally increased or decreased relative to the difficulty of the past two weeks. If it becomes prohibitively expensive for Amazon to mine for bitcoins and they drop out, the threshold will fall again. Either way there's always an incentive to keep it running, until the system is out of coins.
(edit for clarity)
Which won't happen. They exponentially decrease in value per block as time goes on. And to offset this, as fewer are created, they will be worth more, because by that point there will be far more people using Bitcoin (if it doesn't bust, first).
So unless we find every solution to the hash problem, which essentially can't be done, statistically speaking, the system will never stop producing some bitcoins.
So throwing a lot of resources at it just makes it that much more difficult to mine, and would seem not to be productive. (edit: well, at least not based on their current value)