The anti spam claim doesn’t explain why the fee is based on transaction size. A micro fee would discourage spam transactions. If ada goes to 10 dollars then the fee will be 2 dollars just to send 1 
I guess that is due to the fact: there is only IOHK keeping the stake in the network.
No one else can keep the pos. 
By the time we reach 10, I imagine that there will be resources available to make the fee go down.