>And there is some problem with that?
Yes there is. Overgrown forests are available anywhere in the country you want. I can see one if I step out my back door. But Gettysburg, the battlefield, is available in one place and one place only. It's not just another overgrown forest.
Do you want to see it turned into a a housing subdivision with tract houses on quarter-acre lots? The change would be the same as letting it get overgrown -- neither is as it was in 1863.
I would actually prefer the former to the latter. If it is not going to be preserved as a battlefield, then let it be put to economic use. People can hike in jellystone park if they want wilderness.