Yowza. Lots to unpack here.
> most large corporate entities have created more net good than a given forest or lake
"net good?" Perhaps, avoiding the strictly philosophical, you intend to invoke an economic argument wherein the market value of forests or lakes is defined within the bounds of the free market? Such that "large corporate entities'" high economic valuation is "better" than that of a forest? In this context, given that the Mt. Baker-Snoqualmie National Forest outside of Seattle (many trees, many lakes) is valued at $1 Trillion [0], this seems like a bit of a reach. But, even then, why make the comparison, especially since...
> we rely on forests for ... maybe oxygen
Wildlife habitat, drinking water filtration, CO2 sequestration, timber production, refugia against climate change, biodiversity reservoirs
Or, if you like: solitude, silence, wild spaces, discovering one's place in an improbable universe
> from a rationalist perspective, the ideal would be to identify how much forest cover is required and then convert the rest of the land to productive use.
"Productive Use" is a dangerous phrase. There's much to say, but I'll let someone else say it.
In an essay ("The Trouble with Wilderness" [1]) by William Cronon, which I believe you all would enjoy reading, after he finishes dismantling the myth of wilderness in America, he writes: "one of my own most important environmental ethics is that people should always be conscious that they are part of the natural world, inextricably tied to the ecological systems that sustain their lives. Any way of looking at nature that encourages us to believe we are separate from nature––as wilderness tends to do––is likely to reinforce environmentally irresponsible behavior."
Further: "Learning to honor the wild––learning to remember and acknowledge the autonomy of the other––means striving for critical self-consciousness in all of our actions. It means the deep reflection and respect must accompany each act of use, and means too that we must always consider the possibility of non-use. It means looking at the part of nature we intend to turn toward our own ends and asking whether we can use it again and again and again—sustainably—without its being diminished in the process."
> As far as I'm concerned, people who don't live in cities are disadvantaged by their lack of access to corporate support.
As far as I'm concerned, people who do live in cities are disadvantaged by their constant immersion in a world where perverse incentives for corporations (shareholder expectations, government contracts, CEO salaries) define the very environs in which a city-dweller spends their days, rather than the natural world which cradled humanity for hundreds of thousands of years prior to Google and Facebook, to Bechtel and Skanska.
[0]: https://www.wilderness.org/articles/blog/what-intact-forest-...
[1]: https://www.williamcronon.net/writing/Trouble_with_Wildernes...