Who should decide what the fair price of a tx is?
Competition between miners.Sounds good to me! The old supply/demand dynamic, right?
Now, who should decide how much demand there is for tx?
Who should decide what the supply of tx is?
Every miner decides which is the lowest tx fee (cut-off level) for every particular block.
If tx fee for extended period is too low miners will go out of this business.
If tx fee for extended period is too high new miners will enter this business.
What don't you understand?