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.
There is no reason to stick the total reward to 50 BTC because you need to consider the purchasing power.
Here's yet another alternative scheme:
1)
Block size adjustments happen at the same time that network difficulty adjusts2)
On a block size adjustment, the size either stays the same or is increased by a fixed percentage (say, 10%). This percentage is a baked-in constant requiring a hard fork to change.3)
The block size is increased if more than 50% of the blocks in the previous interval have a size greater than or equal to 90% of the max block size. Both of the percentage thresholds are baked in.Example:
A block size adjustment arrives, and the current max block size is 1024KB. The last 210,000 blocks are analyzed, and it is determined that 125,000 of them are at least 922KB in size. The maximum block size is increased by 10% as a result.
Instead of targeting a fixed block reward, this scheme tries to determine if miners are consistently reaching the max block limit when filling a block with transactions (the 90% percentage should be tuned based on historical transaction data). This creates scarcity (in proportion to the 50% figure) while remaining independent of the purchasing power.