In other words, it's easy to talk about a solution, any solution, in isolation of what it would really take to deploy it at a scale massive enough to affect planetary level changes. Think about all of the equipment, transportation, processing, people, energy, etc. We literally can't think about things at these scales.
To go back to my super-simple argument about how the planet behaved without us polluting it further. It typically took absolutely massive storms for tens of thousands of years to reduce CO2 by 100 ppm. I use 50,000 years as an average of sorts, but the reality is that the range extends all the way out to 100,000 years for 100 ppm.
Now imagine taking all of that and proposing that we are going to do the same in 50 or 100 years, without leaving the planet, without shutting down every form of transportation, without killing every industry on the planet and without reducing population to medieval levels. That is a tall order. It's one thing to run a little microscopic test (when compared to planetary scale everything is microscopic), quite another to have the audacity to claim we know what will happen if we take it global. We could create quite an ecological mess.
The things you mention we could have done. I don't think they would have made any difference. I mean, not in terms of atmospheric CO2 concentration. Human life, definitely. One of the things I learned --and I can't remember it accurately-- is that CO2 in the atmosphere "sticks" and is hard to mitigate. The idea is that anything we do at ground level will have zero impact past a certain altitude. We can certainly burn stuff and have the CO2 go up and circle the globe but we can't easily pull it down from 10, 20 or 30 km of altitude.
That's what a lot of these simplistic conversations miss. When I say it will take an astronomical amount of energy and resources I am thinking about a process that has to reach out and grab CO2 going up tens of kilometers into the atmosphere all around the globe. We just can't do this by installing solar panels, sprinkling the beaches with chemicals, banning coal and gasoline powered vehicles and devolving into the middle ages. This is a very tough problem and one that likely has no human-scale solution, both in terms of time and resources.
That said, I don't think this is reason for depression. Imagine for a moment if we agreed on this idea that we just can't fix it. What would our conversations turn to then? Improving human life? Developing technologies to help us live with the coming changes? Cleaning-up our act to improve life locally? That and more.
I would imagine the narrative would change radically. For one thing, we would stop hearing from ignorant politicians, celebrities, deniers and zealots. Researchers would be able to work on real research rather than having to get on the bandwagon for fear of never having access to research grants and opportunities. There is no telling what advances we might make if we just stopped the insanity this has become and allowed facts, reason and real unencumbered science and engineering to float to the top. Utopia, I know.
