You can either get spam that pays MARA through Slipstream and bypasses the p2p network, or you can recognize that policy filters in a centralized mining regime don't work and focus on the main issue: miner centralization. One part of that is: take away their ability to monetize out of band transmission, and stop it from doing further damage to monetary use cases through a lack of visibility of mempool conditions. Fix centralization and we can talk about policy as a spam mitigation tool.