So then this person would respond, "That's great. You and people like you should buy the land, and keep the diversity going. Why is that any of my concern?"

Does not seem like this person is arguing in good faith. Do they never go to aquariums, zoos, or a park? Do they never buy flowers for someone? Do they literally have soylent for every single meal? Do they live in a concrete box on a concrete block with all plastic furniture and clothes? There is a reason why far more people want to live in beautiful diverse places and not in the middle of the desert.

Do they not eat fruit? Drink wine? Drink craft beer?

Apart from that person being an asshole this would actually be the right thing to do. Buy the land and give it to the nature. Deep Ecology foundation used to do just that.

You can't buy the oceans.

Our oceans are collapsing. Warming, acidifying, over-fished, pollution.

What are we supposed to do?

If you follow that line of thought far enough it leads to very dark places.