I do think there are some species that are resource sinks and probably past the point of saving (e.g., possibly the Red Wolf). I believe species like the Black Rail are not yet past that point and serve as a flagship species for larger concerns (e.g., coastal degradation due to overdevelopment and sea level rise). Many other species rely on similar habitat as the Black Rail, so it is not a one-species project.
I don’t really agree with the whole concept of keystone species. No species exists in a vacuum. Managing solely for keystone species leads to a world of generalists with no specialists. There is a much greater number of specialist species (for at least some aspect of their life cycle) than there are generalists.
I’d argue that this is exactly what you’d get when you have a strategy focused only on species humans have labeled “keystone”. Sea otters, white oak, and gray wolves. The American Chestnut had a vast ecological importance, likely beyond that of any single oak species, and while its loss is sorely missed, I don’t think its loss had a greater or longer-lasting impact on biodiversity as a whole compared to any other non-keystone species.
Invasives don’t so much fill a niche as they take it over.
I understand that ceasing all introduced species is impossible. I was not proposing a zero-tolerance or all-or-nothing policy. I do think we can achieve better-than-current practices with a little hard work and resources. Dismissing something as impossible is a poor excuse to not try to make achievable improvements.
I agree preservation is not the answer. This can work on a small scale (e.g., National Parks have a preservationist mission), but does not work on a global scale. I consider myself a conservationist, realizing that it is fruitless to expect to maintain a stasis but that biodiversity is a thing worth protecting and can be done while working with human stakeholders instead of always against them.