I’m cautious about the Miramar proposal because of the reasons you mentioned, but moving MCRD is less of an issue IMO. The real issue with closing MCRD is whether the current airport could even use that land. I am assuming that because it is directly adjacent to the airport and is almost the size of the current airport, that they could. I’m no expert though.
I am an expert about Camp Pendleton though as I served there for a number of years. For starters, last time I checked over 5000 acres of the base were leased to state beaches and agricultural fields. The agricultural fields alone could probably house numerous MCRDs. Environmental issues also limit alot of what can be done at the base, and current training does take up a large portion of the base. So I would never advocate putting the airport there, but integrating MCRD into the area of the base that already does recruit training, or somewhere else, would make sense to me.