>I just wonder if humanity’s adventurous nature is leading us away from a proper focus on the sustainability of our civilization, our specie, and our fragile planetary environment?
But we still need spaceflight at least for planetary defense against asteroids, mining asteroids(so we don't have to mine Earth), etc.
With chemical rockets, not much.
With "a propellant-less propulsion propulsion system such as solar sails or electric sails," bringing water (propellant) to low-earth orbit starts making sense [1], as does mining platinum, but only if "the quantity of platinum from space would substitute an equal quantity of terrestrial platinum," i.e. moving heavy industry off the Earth's surface [1].
Given asteroid-mining profitability is dominated by "the throughput rate, which depends on the mining process," it's possibly to see a path to certain rare-earth minerals becoming profitable to mine in space if environmental controls on Earth are tightened while constant-thrust propulsion technologies advance.
The energy involved in chemical rocketry is actually not that much. Getting a kilogram to LEO is roughly as expensive (in energy) as flying it to the other side of the world in an airliner. Getting stuff back from an earth-crossing asteroid can also be very cheap energetically, with very small delta-V (if one is willing to wait long enough).
Also, the materials we're talking about from asteroid mining, like platinum group elements, probably are shipped by air, just for security.
This whole argument is reminding me of the facile and bogus argument that launch to earth orbit from planet's surface is expensive because of the energy needed.
So far we have... "maybe platinum." Maybe!
Aside from the conspicuous absence of math, "maybe platinum" isn't remotely important enough a factor in earthbound mining to justify asteroid mining on the basis of preserving earth, obviously.
By moving mining off planet to the moon and asteroids we eliminate the carbon emissions and environmental damage that occurs from these processes.
So how do we do it? We don't need to start sending fleets and fleets of crafts into space which is energetically expensive and time consuming, what we need to do is develop self-replicating mining equipment that we can send to turn the things we want to mine into more things that can mine for us. We won't achieve full self-replication immediately but we can definitely send machines that can do partial self-replication at first and work on improving the ratio of material sent to material returned.
What do we send back? titanium, platinum, aluminum, nickel, iron, whatever we find there. If platinum was as cheap and plentiful as aluminum was now we would have all sorts of catalytic uses for it that would significantly clean up our Earth based industrial practices.
How do we send it back? If we're sending it back from the moon that's easy -- a space elevator or rail gun. If we're sending it back from asteroids inefficient but cheap iron ion thrusters. Use iron from the asteroids as fuel.
Bonus points: Send it back covered in ablative material like mined olivine that can react with CO2 in the atmosphere to sequester it, that way the whole process isn't just moving environmentally damaging industrial practices to other solar bodies but we're undoing some of the damage from past human activities.
... but this won't happen unless it's economically advantageous, with all risk, transit, and upfront investment included
We already don't have the political will to force single-digit increases in cost of output to reduce environmental damage. Why would we then?
Not too sure! Probably one part is to disabuse people of the notion that we have alternatives available if only we could just dedicate more brainpower to space travel.
The real answer is to eliminate the tradeoffs through scientific innovation. As mentioned elsewhere, the real way to protect earth (and our species) is to produce better plastics, proteins, and pesticides. Each smart person wrapped up in sci-fi fantasies of simply off-planeting heavy industry is another brain dedicated to -- as far as I can tell (thus my questioning here) -- an almost completely useless endeavor.
Which is why I believe the aforementioned disabusal is critically important. There's just no evidence that big explosions and adventures are what we need. We need people in labs nudging molecules over and over with a 0.0001% success rate.