A problem with a fixed btc fee is the price of bitcoin. If the price would drop an attack could happen as it would be cheaper. If the price rises then legitimate users would be affected negatively. Percentage fees lead to a similar problem with bitcoin losing its edge over the banking system.
After all, wouldn't this problem be solved - at a cost of storage space - with increasing the block size?
i agree with you bitcoin were made to take out those killing fees that banks takes for any thing we wanna pay or tranfer to others,if we raise it we will come to a point that bitcoin would loose all interest since use it or bank will cost the same,on those time bitcoin would be dead.