It's a fundamental problem with almost anything to do with the state (in the general sense), including budgets. Anything that is discretionary will eventually be raided by an opportunistic politician and binding future policy makers is very difficult, if not downright impossible.
As long as the general rule of law is strong with robust property rights, however, creating a new private organization like a trust with a strict charter that limits its future stakeholders is a lot easier.
It's not just the state. All human institutions can change. I hope these nonprofit land conservancies can continue their mission forever, but it's not guaranteed.
Hell, look at monasteries. Several societies with large religious followings couldn't prevent the dissolution of monasteries - Tang Dynasty China (cutting down Buddhist ones), and Tudor England (cutting down Christian monasteries).
As long as the general rule of law is strong with robust property rights, however, creating a new private organization like a trust with a strict charter that limits its future stakeholders is a lot easier.