The idea that Trump, or any president, has the legal authority under the Antiquities Act to undo or significantly alter monuments designated by his predecessors is legally doubtful, at best. Thus any orders built on any such bureaucratic review were certain to be tied up in court for years, at least, as monument supporters fought back.
That might offer a glimmer of hope to people who oppose the whole idea of national monuments, to those who hold out a mostly irrational hope of an economic bonanza sparked by a land-rush of access to a lot of coal, oil, natural gas and grazing land that either isn’t there or isn’t profitably exploitable.