The short answer is this: The block reward is not meant to be a permanent part of the system. It is only there to bootstrap it. It's designed to slowly fade out

The short answer is this: The block reward is not meant to be a permanent part of the system. It is only there to bootstrap it. It's designed to slowly fade out, at which point a fully self-contained market of tx-fees will have been established, which works according to the classic rules of price-finding by demand and supply.