Not a single correct thing in that, I believe! Life on Earth has evolved to live in hundreds of dissimilar environments. Heavy salt is toxic, yet fish manage the ocean. Yellowstone has life in mud pots at hundreds of degrees. Air doesn't exist underground yet bacteria thrive there. Hell, there are living things on the surface of the space station.

To get people to tolerate more salt, we'd have to have a fish's ability to regulate it. Ok, genes for that. Air issues? Filters, microstructures to migrate particulates out, baffles (like many creates already have). Deep-ocean pressure? Only a couple of issues there, and certainly life thrives at the bottom of the ocean.

It's not optimism; its simple observation of what we have already on Earth in existing life forms.

And 30,000 years changes not a thing. The log of 1 Trillion base 2 is 39. In 40 generations of space colonization, that's 40 times 30,000 times however long it takes to rebuild the capability on each colony. In a dozen million years the galaxy would be covered. Its 20B years old. So, nobody's tried it yet.