I think that the blockspace forum's argument makes sense. In particular, it sounds like the "latest" selection rule *has* been enforceable: "Historically, some validators forked or used software that allows them to select the highest bid and started to observe lower rewards or received warnings from builders that if they continued they would receive less / less competitive bids." This makes playing "latest" rational for the large stakers, because you will be asymmetrically punished for playing "highest." If the default becomes everyone plays "highest," then the builders lose their ability to asymmetrically punish and are forced to shade. From there, it is certainly true that the large staking pools will have the ability to credibly commit to honoring cancellations and thus receiving unshaded bids.