Earlier quoted context omitted.
Steel and concrete aren't necessary for the most part: other materials that don't emit (or even actively sequester) carbon are available and have been for a long, long time. It doesn't even preclude building densely either: just look at cross laminated timber.
We're going to build bridges and trains and rails and roads and ships out of cross laminated timber? And yes, you can build pretty tall structures out of CLT, but there's still smaller than what you can build with steel and concrete. The tallest one built that I know of is 25 stories. But at least 6 or so of those floors are all concrete+steel, and it still uses concrete+steel emergency stairwells. https://www.fs.usd…
https://www.geoengineer.org/education/ground-improvement/lim...
As for the rest, I was referring to buildings. I don't think anything is so worthy of being built that wrecking the environment is a fair trade. Railways are an honourable exception, since they displace (far more wasteful) cars. Light rail is especially good for this.
The fact is that we need to question what we build, as well as the materials used to build it.