The argument used to be that rising transaction fees would balance dropping mining rewards. I haven't heard that so much lately, probably because people are already not actually transacting in bitcoin due to the high transaction fees.
it's a bit of a chicken and egg problem. High transaction fees should attract more miners (who might be willing to accept lower transaction fees due to competition). This ought to reduce transaction fees to the market rate.
However, because there's so little real demand for bitcoin transactions from consumers, that this market rate isn't really a proper market rate. It needs to grow in volume. But the volume of bitcoin transactions is too low (by design).
So to get more bitcoin transactions to lower the rate, you first need more bitcoin transactions, but it's too expensive and consumers don't use it!
