The United States went through a roughly 100 year period or so where conservation efforts were... er, not terribly great. It took about 100 years for humans to shoot an incredibly large passenger pigeon population into extinction, and bison which once numbered millions on the plains was reduced to mere hundreds. Fortunately, in the mid 19th century to early 20th century in particular, people began to notice and started protecting areas. Thank goodness for that, but a large portion of ecosystems did get wiped out (to give one example, roughly 95% of the California redwood forests).

Megafauna do get most of the attention, but a lot of the hidden economic / scientific benefits of maintaining biodiversity I think comes from smaller parts of the ecosystem -- for instance, drug discovery. Or, the widely used polymerase chain reaction (PCR) technique for copying DNA... which for all we know is a technique that might not even exist today had preservationists not had the foresight to preserve the Lower Geyser Basin of Yellowstone National Park, where the crucial enzymes for this technique were found.

Unfortunately that period also roughly corresponded with the rise of the middle class and lifting the majority of the population to a level of material comfort that was (at it's peak at least) the envy of the world.

Most of the worst environmental damage today is not happening in the highly developed west, it's happening in the rest of the world that - understandably - doesn't care as much about saving the local animals as it cares about catching up to the material standards of the wealthy world.

> Our living environments create opportunities for well-adapted species to flourish

That statement is a tautology. "Some" species will flourish in all productive biomes, by definition. But the ecosystems that existed will be highly disrupted. So we're replacing large predator marine species with jellyfish. Beavers with rats, wolves with feral cats, etc... And the plant diversity is quite a bit more impacted.

Your point amounts to "well, all life isn't going extince, so what's your problem?". The problem is that there is moral value in preserving what exists. Some of us like to believe that "causing a mass extinction event due to entirely preventable causes" is, y'know, a bad thing.

I don't think it is a matter of what humans can do if they make enough of an effort, but what is most likely to happen.