Essentially the point is to be self-limiting. No one is saying "Planets outside of these parameters can't have complex life." but instead "Planets outside of these parameters could not give rise to what we know as complex life."
Why would life depend on the low concentration that is not even toxic to all of the life here on Earth?
It wouldn't, and the study isn't saying that it would.
The point is about narrowing the search space. An analogy would be guessing passwords. If I want to guess the passwords of the people I work with I can start with the assumption that it could be anything, which doesn't narrow the space and makes the task impossible, or I can start with the assumption that it'll be based on a dictionary word with some numbers because all my passwords are like that. I'll definitely miss some passwords that don't fit my assumption but I'm far more likely to get some positive results (assuming my way of generating a password is common in the universe.)
Take the CO criterion, for example. While it is reasonable to point out that CO may be present in high concentrations in many planetary atmospheres, at levels that would be toxic to life evolved on earth, one must consider that life might evolve in a way that tolerates or even exploits it. Oddly, the authors acknowledge this, yet, apparently without any counter-argument, still include CO as a probable show-stopper.
As the way to search for life beyond the solar system is to look at atmospheric composition, an alternative way to look at the 'CO problem' is to say that an abnormally low CO level on these planets might be a hint of life.
https://phys.org/news/2018-06-advanced-civilization-universe...
Given the amount of real estate in the universe that is "out in the cold" (Oort cloud equivalents, way beyond the snow line) it may be that we are the oddballs, and there are scientists out there wondering how life would be possible with molten ice and burning things with something as hellishly reactive as atomic oxygen. Free-floating planets might be where the really good parties are in our galaxy.
Outside of the matter that we are made of, who knows. It's not impossible that dark matter could somehow be set-up to create life, but we've no idea if it's feasible. The same goes for dark energy, only more so as we have basically no clue what dark energy is, even though it makes up the majority of the 'stuff' in the cosmos.
[0] https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...
When life started here CO2 was from hundreds of thousands to many millions of times more abundant than it's now. Tens of thousands more of it won't probably even make oxygen respiration impossible.
https://en.wikipedia.org/wiki/Hydrothermal_vent#The_Deep_Hot...
Other extremophiles and archea exist and have been observed in deep caves and rocks. Life really does find a way!
I guess isn't silicon the only other that bonds four ways? Not a chemist but I feel that 4 bonds are a big thing.
https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi...
Between them, carbon and water have a bunch of unique properties. AFAIK, its hard to come up with any other suite of materials offering the same prospects for complexity, at any temperature.
Also I don't see why you couldn't have an organism that "breathes" solid material.
> Silicon dioxide melting point: 1710 °C
No complex chemistry can survive these temperatures. SiO2 is only soluble in HCl and HF, which are very reactive, so no solubility either.
> Also I don't see why you couldn't have an organism that "breathes" solid material.
All of known biological chemistry relies on some sort of solubility for reagents. I don't think there's any known organism that can get rid of solid waste from the entire volume of the organism.
You can also have chlorine as your oxidizer and make Silicon tetrachloride which melts at −68.74 °C and boils at 57.65 °C, that temperature range is perfect.
What else will change by having a chlorine instead of oxygen atmosphere?
Life didn't evolve to breathe nitrogen, oxygen, etc which we just coincidentally had - it evolved to breathe and utilize what was already present on our planet. And this should be the assumption for all life, which means that restricting stuff because it would be inhospitable to us is completely nonsensical.