I don't want all land in its most productive use, and neither, I suspect, does the rest of the world.
If we want a thing, we should subsidize it directly rather than as a side-effect of a mostly bad policy.
It's true that this problem doesn't arise under Georgist tax policies (AFAIU) for public land, so there's that. But I still feel a little uneasy about making land preservation by individuals prohibitively expensive.
Only if that land preservation is in the middle of the city ("land preservation" that typically looks like gated-in parking lots). Nature access can and is subsidized in the city.
Someone can maintain a piece of land unproductively if they want to. They just have to pay for that right, same as anyone else that would want to own that piece of land.
There is a broad range of situations here. No one should want a blighted property just sitting there in places like NYC or LA where homelessness is rampant. However, no one is arguing to just teardown places like Central Park to put in more housing.