But filters **_could_** make it very difficult, IF mining was sufficiently decentralized. The cost of out of band tx's should reflect the probability for a miner hitting a block. If mining is decentralized, that probability is low. If blocks are full, then the cost of an out of band transaction is that much higher.
Decentralizing mining is the real fight. This move seems like they're trying to make that fight irrelevant, like someone is afraid they're about to lose. Is it coincidence that the Knots guys are also running Ocean? It doesn't look like it to me.