There’s no planet B

I know of precisely zero astronomers who consider Mars to be Earth's identical twin. Usually that's said of Venus, not Mars.

> The proposed absolute best-case scenario for terraforming Mars leaves us with an atmosphere we are incapable of breathing

That ain't the proposed "absolute best-case scenario". The actual best-case scenario comes from the steps after thickening the atmosphere with carbon dioxide - namely, the "grow plants in an atmosphere wherein they can actually breathe and not immediately freeze to death or get zapped to death by cosmic radiation".

Also, carbon dioxide ain't the only gas that would be involved. Water vapor is just as crucial. Neither are sufficiently abundant on Mars to be useful for terraforming, but asteroids tend to be rich with hydrocarbons and water - and NASA has already been testing the feasibility of redirecting asteroids.

> a more realistic scenario might be to build habitat domes on its surface

Outside of sci-fi that's rarely proposed. The more typical proposal (besides smaller, non-dome habitats) is to build underground. Easier to shield against radiation, easier to mitigate leaks.

> for example, this concept does not consider plate tectonics, which are thought to be crucial to sustain life on Earth.

Thought by whom? The link doesn't seem to assert that, and there's no reason to believe that plate tectonics contribute in any meaningful way to sustaining life on Earth. They likely contributed to creating life on Earth, but last I checked I ain't a thermophilic microbe huddled around a geothermal vent on the ocean floor.

> Earth’s current aeon, the Phanerozoic, began only around 541 million years ago with the Cambrian explosion [...] In 1 billion years, the gradual warming of our Sun is predicted to cause Earth’s oceans to boil away. While this certainly sounds worrying, 1 billion years is a long, long time.

The author's inconsistency on whether time periods are relatively "long" or "short" depending on whether it helps or hinder's the author's argument makes it increasingly difficult to assume the author is arguing in good faith.

----

While I agree fully with the article that the destruction of Earth's biosphere can and must be prevented, I disagree strongly with the article's implicit conclusion that expanding beyond Earth is not a component of that. Earth is already straining to support the human population and its modern standard of living; that'll only get worse as the population grows, as those currently living in poorer parts of the world rightly demand a standard of living closer to that of the richer parts of the world, and as the richer parts of the world push the expected average standard of living yet higher.

We need to get as many humans off of Earth as we physically can, before Earth evicts us. We need to be moving as close to the entirety of our industry as possible into orbit, the Moon, and beyond. We need to give Earth an opportunity to heal from the damage we've done to it - and we need to buy time until we have the technological means to affordably do so.

That doesn't seem all that unreachable, honestly. The average middle-class income could swing that with a payment plan (said incomes commonly cover car payments already), and even for those who can't afford it, an enterprise seeking workers could surely cover that as part of an employment contract. We could also probably slash the costs - and multiply the scale - even further with non-rocket launch systems like skyhooks or launch loops.