Also, requiring a total reward of 50BTC means requiring 25BTC in fee NOW. As the typical total tx fee in a block is about 0.25BTC, the fee has to increase by 100x and obviously this will kill the system.
How and why would the system be "killed"? The max block size would simply not increase.
So you propose that the size will only increase but never decrease? Anyway, the major problem is the choice of total reward amount because change in purchasing power is unpredictable.