As your friendly, currently doing repairs around my home National Park ranger, here is the deal. The National Park Service is charged with protecting, preserving and interpreting the National Parks. That charge, as well as the designation of most National Parks (putting aside National Monuments for a moment) comes from Congress. So when they pull the plug on funding, the gates get locked because that is most effective way to protect and preserve the resources when no staff is around. A huge portion of the budget goes to preservation and restoration purposes repairing the damage of jackasses who don’t get it.
If the gates were open right now with only about 12% of the staff still working, I can assure you that the battlefield sites would be packed with people with metal detectors and shovels taking away your heritage. And others would be hacking more convenient trails through sensitive areas, giving no thought to the fact that trails are often sited specifically to avoid erosion damage to the cool things you are coming out to see.
As for concession revenue, it does help but it is a very small percentage of the NPS budget. And many parks do not have concessions beyond the small bookstore/gift shop - and trust me, the small cut we get from those is not a difference maker.
There are 401 National Park units across the country, as well as other programs. Each one has been designated by legislation as being nationally significant. We don’t get to pick and choose which ones are “really important.” They are all important - and irreplaceable. So to the Congressmen who are furiously masturbating over World War II vets down at their Memorial, walk back up the hill and do your job so that all of the Memorials can be reopened.
Oh - maybe a few of you could wander past the Korean War Memorial on the way back and lay a wreath. It is the 60th Anniversary of the end of that war. Those poor bastards always get forgotten.