Having a fee is a great way to avoid having to many unnecessary transactions but the miners don't need the fees to profit right now, the block reward is enough so even lower fees in average could be possible.
It's true that miners don't need the fees to profit, but then they also don't need the transactions to profit. Indeed, it seems that the block reward presently works against "even lower fees" because a transaction included in a block increases the probability that the block will be orphaned.
Could you explain why a transaction included in a block increases the probability that the block will be orphaned?
Sure.
Currently, when a new block is found, the entire block is sent to adjacent nodes which then, in turn, pass the block to their neighbours. Meanwhile, other miners continue to build on the previous block, likely unaware that a new block has been found. This delay between finding a block and getting a copy to every miner is what gives rise to orphans.
Including a transaction in a block increases the size of the block. A block full of transactions can be a thousand times larger than an empty block. Larger blocks take longer to propagate throughout the network and so allow more time for rival blocks to inadvertently appear.
You might be interested in some
rough calculations concerning this effect Gavin did a little while ago.
You might also be interested in
this proposal to improve the efficiency of relaying blocks.
