I understand the frustration and heartbreak, even though I've never played, or even been close to, Carly's Playground.
There is always this risk when putting courses on somebody else's land---whether a city park, a private property owner, a commercial property, or somewhere like this. The deal doesn't include "forever", and the owners may have a change of plans. About all you can do is hope they like having disc golf there enough that they want to keep it.
Even if disc golfers ponied up the money to buy land and build a course, there would be no guarantee of its permanence.
I wish you well in pursuing a replacement. Wherever you end up, I suggest you don't tell the owners of the prospective property that, if they allow you to build a disc golf course, then a decade or two from now, if they want to do something different with the property, they can't.