Since 2014, a large portion of the San Gabriel Mountains are now part of a national monument. Don't have a map handy, but it includes most of the major peaks, and I think both portions of CA-2 and CA-39 are within. The former is presently closed from roughly La Canada to the Big Pines Highway due to a rock slide, and the latter has been closed from the CA-2 junction down to Crystal Lake since 1978.
My question is, now that the mountains are a national monument, how does that affect Caltrans' ability, if at all, to maintain these two highways? Does maintenance switch to the National Park Service? If so, any likelihood that CA-39 will ever finally be reopened and connect to CA-2? I've read that Caltrans has tried to relinquish the entirety of CA-39 since 2012 but old laws have largely prevente them from doing so (i.e. they'd have to literally deconstruct the entire roadbed).
CA 190 and CA 178 still exist (both legislatively and in Caltrans' logs) within Death Valley Nat'l Park. The highways were there first, before the park was created. I would think the same would happen for a newly-created national monument, which is a lesser breed of Federal preserve.
Environmental laws' protection of parks of whatever kind would almost certainly block the re-completion of CA 39 through the mountains if there wasn't already a closed road there. Reopening a closed state highway would be different, though I don't know how much different.
Also keep in mind the National Monument is a USFS-managed site and not NPS or BLM.
Here is a tour of the abandoned part of CA-39 by Sidetrack Adventures.