Who is paying the spammers and how long can they support the spam? Eventually the spam has to provide value to someone, otherwise it will get priced out by those producing real value and storing/using that value in/on the bitcoin network. Im storing my work on the network and will pay to move it to get something I value. I will outwork the spammers on a long time window and will be able to afford fees. In this way the the fee market + humans working filters out less valuable non-monetary uses.