My whole opinion on terraforming Mars is it’s a question for the indigenous Martians, not for Earth. Once there are people on Mars (perhaps descendants of people who come in our lifetimes) who have lived and survived on the planet for long enough for Mars to be their home, for them to have developed a distinctly Martian culture, way of life, and possibly cultural/mythological/spiritual/religious connections with the planet and their surroundings, who have had to survive the harsh environment for at least a few generations, then THEY should make the decision, not people millions of miles away arguing for or against it.

They are the only ones who will have sufficient stake (their own lives) and perspective (living on the planet for the vast majority of their natural lives) to make this decision.

Just like I think indigenous people on Earth have the right to environmental decisions on their land, so should Martians. They will cherish Mars more than we will ever understand.

Secondly, I think that terraforming Mars should be seen in two levels:

1) making the atmosphere thick enough to go without a pressure suit in the deepest areas. This is technically possible (although not comfortable or healthy or safe, and requires some positive pressure) once the pressure exceeds the Armstrong Limit, or 0.9psi, in Hellas Basin. Notably, this can be even achieved (with a safety factor of 2!) within the volatiles budget that the “NASA says terraforming Mars is impossible” Jakosky paper claims. ...which is also something really frustrating. The bar for what counts as “terraformed” is made very high by the authors, higher than the physiological reality, and this is somewhat arbitrary but they treat it as an objective fact. Anyway. 1-2psi or so, mostly CO2. Similar conditions to this actually persisted for a while on Mars during the Noachian Period where there was flowing water and a very active water cycle, and this may have been when Mars had life that thrived if there was any. This level of terraforming could be accomplished within a century in principle. So if anything, this level of terraforming is really “Areoforming” and would potentially allow Martian life to thrive. This level is achievable and makes Mars settlement MUCH easier and safer without insane energy requirements (just orbital mirrors or solar sails could do it). Martians may decide to stop at this point.

2) actual breathable atmosphere. The pressure would have to be higher, probably 3-5psi, and oxygen to 1-3psi partial pressure. It would change Mars by oxidizing the surface. But the energy required to make this much oxygen is insane. petawatts for centuries. (Humanity uses just a few Terawatts of useful energy right now.) it’d likely kill surface Mars life unless it was adapted. And by the time this happens, Martians may already have adapted to the low oxygen environment, with maybe some sort of mechatronic/prosthetic oxygen carrying organ just like whales adapted to the airless ocean depths. So it may just be unnecessary. (There’s one trick that could possibly make this fast and cheap: if huge quantities of perchlorates are found. Perchlorates exothermally release oxygen when catalyzed... they actually could produce power with oxygen as a byproduct. Some oxygen candles use perchlorates, and perchlorates have been found to be fairly common on Mars. It’s a hail-Mary, but is an interesting potential option.)

The timescale and feasibility (and, I argue, ethical considerations) between 1 and 2 are orders of magnitude apart but they’re often treated as the same exact question, especially in the press which doesn’t like getting too technical.