I don't think it's true that "details can't be shared(because they don't exist)". It is a political imperative for Republicans to repeal the ACA now, and worry about replacing it after the positive PR. Without doing that, Democrats have no reason to come to the table to work on bipartisan reform (not that Republicans necessarily want this), and any replacement will be spun as a minor modification to Obamacare, which causes big political problems for the GOP both among its base and among swing voters. Details of a replacement aren't being shared right now because it would be tactically damaging to Republicans and Democrats don't want to attempt reform.