You know, considering that one purpose of the coin drops is eventually to get to zero and have everything be running off of transaction fees, perhaps some adjustment to the transaction fees should be made near these jumps so as to get some (not all) of the lost-gen in as increased fees? I know that I hate paying fees myself, but if a block only generates 0.1 BTC for whoever creates the block then we're not gonna see much difficulty by the time we get to that point.
Any drop in difficulty when we get to 25 BTC I don't see as that problematic, it's when we get to the lower levels that I'm more concerned about long-term damage.
No matter how you structured the fees there simply isn't the transaction volume to support the 25 BTC which will be removed. The developers have expressed interest in modifying fee structure and IMHO the current structure is simply unsustainable but fees don't matter if the volume is so low.
http://blockchain.info/charts/cost-per-transaction-percentThe cost of a transaction (w/o subsidies0 is roughly 10%. So if today all block subsidies were eliminated to support current network hashing power at current prices would require a 10% fee structure. Note that is a fee on both transaction and change. So actual fee would be much higher maybe 15%+.
The goal should be to grow the network. With 10x the transaction volume it would require a fee of 1%. With 100x the transaction volume it would require a fee of 0.1%.