If it gets zoned as parkland as part of a sale - great! You should be able to make that part of a sale contract. But if the governing body then votes to make it something else a decade later, that should simply be part of how things work.
Old people ossifying things to how they prefer via preventing future generations to freely operate is not how I want a society to run. If anything the older you get the less say in the future you should have.
It gets nuanced - but in general speaking terms this sort of thing should never be forever set in stone because someone alive 100 years ago decided as such via a private contract. Many other ways to go about setting aside areas for conservation.
Even conservation trusts make more sense to me. It’s still private, but they have an incentive to stay receptive to public comment and be a bit flexible. They might swap that 10 acres for another 100 acres somewhere else that creates a 1200 acre contiguous wilderness or what have you in order to stay relevant to contemporary needs while still staying true to the 250 year old mission.
I simply do not think you should be able to dictate (via private means) what happens to a property after you sell it. That’s for the next person who owns it to decide - in accordance with current local zoning and land use guidelines.
I see plenty of people here angry when the idea is floated of the US government opening up public land for mining, drilling, etc. You may not be one of them obviously, but how is this different?
What do you think the outcome of this would actually be?
Someone wants to sell land to develop a parkland but they aren't allowed to dictate that it must be a parkland.
So they just don't sell it ever. Now instead of a nice park it's a direlect lot for decades
The answer to this problem isn't "fuck you old people we're taking your land and building data centers"
Most contracts are legally mandated to have time limits. I think that's a good policy.
In this case an explicit number of years it has to stay a park would probably work better than an attempt at indefinitely defining the land.