An example
Being specialists in walking holidays, let’s take footpaths as an example.
Going on a holiday that uses a network of ancient mule tracks is sustainable as it means that these trails, which were used by locals for centuries but which have fallen into disuse with the advent of motorised vehicles, do not get overgrown and forgotten.
On the other hand, walking along a badly eroded path for the sake of reaching an ‘unmissable’ viewpoint is unsustainable, as there will come a point where it becomes impassable and no one will be able to admire the view.