Yes they can. The computing power necessary to secure the blockchain will always increase, thus at some arbitrary point in the future, the difficulty of currently generated coins will be "too low".
If the reward (50BTC) were not an arbitrary constant, but pegged logarithmically to the difficulty, that would even this disparity out some even more...a proposal I have suggested in the past and seemed relatively palatable (at least compared to the strong sentiment against "don't mess with our block chain")
The reward (per miner) is pegged linearly to the difficulty right now. Why would logarithmically be better?