i think proposers are always necessary. The simplest reason is that if someone wants to build a block locally and not use the external block building market, they should be able to without penalty (i.e., we don’t want to ~force~ people to use ePBS). Another reason is for censorship resistance. IIUC these designs rely on having an honest proposer specify a set of txns that must be included. Without the proposer, the builders have no constraints placed on them.
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