Will Bitcoin suffer from a mining Tragedy of the Commons when mining fees drop to zero? One issue that I haven't seen clearly addressed is the possible Tragedy of the Commons in Bitcoin's future (see this answer). 40 years into the future, the only significant direct benefit for honest miners is transaction fees, as the block reward is almost zero.

The potential problem here is Tragedy of the Commons - it is possible that while the entire network will benefit from miners, the financial incentive for any individual honest miner to mine will be too small, so fewer miners "than necessary" will mine, leaving the network open to 51% attacks. In this scenario, the Bitcoin network will fall pray to frequent attacks and will effectively disintegrate. Also consider extensions like Open Transactions, that might make conducting transactions on the real Bitcoin network unnecessary for most users, who will flock to 2nd level networks and reduce the miner's rewards from TX fees even more.

As Bitcoin is being designed and promoted as a Future Proof system, people have tried to analyse the system's economic behavior in this future.

What analysis has been done on this problem? Is the problem likely to happen (speculative question, I know ... but back up your arguments)? If there is a likely problem in the current implementation, what are some possible way to adjust the protocol in the future to combat this problem?

Update - please see here, I described a possible scenario that leads to network insecurity because of this issue.

Update 2 - linking to a recent thread I created on the forum about this.

Update 3 - Added a wiki entry.
There will definitely be a tragedy of the commons problem if things stand as they are now. This was discussed at some length here and elsewhere.

There are some proposed ways to address this and make transaction fees nonzero (block size limit, hardcoded fees, insurance entities, mining cartel, gentleman's agreements which are maintained for fear defection would beget more defection, etc.), but I don't think either results in the combination of competitive transaction fees and a level of hashrate which is enough to offer reasonable security.

This means that there must be some way to improve the network's security at a given hashrate. The only way I know to do that is by augmenting proof-of-work with proof-of-stake, discussed for example here. This is based on the observation that obtaining a majority of the bitcoins in existence is orders of magnitude more difficult than obtaining a majority of the hashrate.