Just leave areas alone and let nature do its thing.
Human intervention destroyed the ecosystem and there is immediate and ongoing harm to that destruction. It's idiotic to suddenly decide to "let nature do it's thing" now when billions of people are threatened by climate change.
I think there's a false dichotomy about what "human intervention" means, because as a civilization, we think of human intervention as forces that act upon the ecosystem from the outside. Not all cultures and civilizations sees it that way. I think we think that way in modern times because we see ourselves as apart from the ecosystem rather than a part of the ecosystem. It's pretty common to hear people say, "there's nothing there", even if it is a lush, wildland (or close enough to it).
As an example, there are ancient, abandoned perennial food forests that were created by the inhabitants. These work with natural processes so that yields useful to humans are byproducts of the healthy ecosystem, and they can carry on even when humans stop tending them.
This isn't really about saving the planet. Over a large enough timescale, the planet will recover just fine -- as a living system, the global ecology can and does regenerate.
Instead, what we're fighting for is a place for humans within the global ecosystem.
"Human intervention" need not be destructive. It has been destructive because we came to see ourselves as apart from the ecosystem, rather than being part of the ecosystem. There have been cultures, to one degree or another, see themselves and conduct themsleves as part of the ecosystem, and tended the lands as such.
Wastelands can be remediated. In India, there are people working with marginalized castes and ethnic groups to restore wastelands. It's not something that works over a single season, but it is something that starts yielding stuff useful to humans within a single generation. The work that these people do ends up restoring land from bare rock eroded by poor land management practices. These were land no one wanted, and now they are both productive (yielding something useful) while at the same time, a fertile, active ecosystem beyond the human-centric view of the world.
There doesn't have to be a dichotomy between "human intervention", and "leaving nature alone". We're a part of the natural ecosystem -- how could we really ever "leave it alone"?
So the "just" part of" just leave areas alone, is asking quite a lot. That's not to say it's wrong ... but it's more ambitious than e.g. net-zero goals. In some sense, it's exactly 2x as ambitious as E. O. Wilson's "Half-Earth" proposal.
There are a lot of trees and invasives. Sometimes if we plant trees and give them a few years of active removal of the invasives they will grow strong enough to out compete the invasive, but if you don't remove the invasive they will be chocked out and die. However only sometimes.
I think it is more about how many ecological relationship something has with other species within the local ecosystem.
Something without a lot of interactions would just take resources and contribute nothing back to the ecosystem would degrade an ecosystem until there are adaptations to include that species within the ecosystem. Contribution could be food for other species. They might provide habitat. Maybe they help spread seeds, or distribute nutrients for plants. And yes, yields for humans too.
By that understanding, I would say that the modern human is probably the most successful invasive specie across many ecosystems.
And it isn't that, I am advocating that we get rid of the humans within those ecosystems, but find ways for humans to participate within that local ecosystem. Once those relationships are established, it is better for that local ecosystem to have humans than to not have humans.
The way things are going we're going to need to learn how to construct artificial reefs to survive as a species, and quite possibly also artificial game habitats. Planting trees is just one part of that.
This is an accidental grove out along the CAS canal project. The bearm made unintentionally by the canal designers captured monsoon rains and accumulated organics for a grove to naturally spring up there. https://youtu.be/jf8usAesJvo
The more impressive (deliberate) projects involve turning barerock wastelands into productive lands using these principles.
There is a big difference between human intervention that sets us apart from the ecosystem (in which we act upon the ecosystem) and human intervention where we work as a participant of the ecosystem (in which we participate within the ecosystem). We can and should use technology, but it is a different paradigm and a different way of seeing.
Some of the things the permaculture folks have been quietly doing for over 50 years have yielded some impressive results. They go far beyond what you'd hear about in the mainstream, and we're only seeing some of it trickle in (such as the ideas of restorative and regenerative agriculture or agroforestry).
A good short video about how forests harvest maximum sunlight, and how many other things that are good for the environment as a whole, happen effortlessly in them, due to all of which they keep getting better over time:
Watch "Permaculture - from forest to farm | Clea Chandmal | TEDxBITSGoa" on YouTube