I've been thinking about this, and I haven't figured out yet how this is going to work. In fact, I don't think I understand the mining economics of a future Bitcoin-system that is dominated by transaction fees instead of coin generation. What would prevent a "race to the bottom", where miners ask lower and lower fees, to out-compete competitors?
The few miners who happen to have the most efficient computers and the lowest electricity prices will be able to out-compete all others. Once others leave the game, the difficulty decreases, but the ratio between the remaining miners doesn't change. The few successful ones can continue lowering the transaction fee, so that the game remains unprofitable for the other ones. In the end, there is a real danger of a 51% attack.
I think that, currently, the only thing that keeps difficulty high is the high value of the generated bitcoins per block. With the generated coin value dropping to zero, I think difficulty will also drop to zero, and the system will fail.
Luckily, it will take several decades before this happens, and most in-between lowerings of the amount of generated bitcoins will probably be over-compensated by the increase in value due to increased usage.
That's an awful lot of assumptions between now and the system failing in a few decades.
but sharp assumptions.
the effect you are talking about is discussed as "tragedy of the commons" for example here: https://bitcointalksearch.org/topic/bitcoin-tragedy-of-the-commons-67900
and here: http://bitcoin.stackexchange.com/questions/3111/will-bitcoin-suffer-from-a-mining-tragedy-of-the-commons-when-mining-fees-drop-t/3129#3129
but with namecoin there is not much to be done about this anyway - it works just like that.
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