I agree that industrial civilization is very likely doomed. Or at least, except for the wealthy. There's just too much momentum from CO2 already in the atmosphere. Even if no more were released, starting today. And then there are all the potential, arguably likely, positive feedbacks. Such as melting of permafrost in northern Canada and Siberia.
But I disagree about the mass extinction aspect. Earth is in the midst of an anthropogenic mass extinction. But collapse of industrial civilization will mitigate that, I suspect. Climate change will drive some species extinct, but overall diversity will do better without so many people.
As you say, CO2 levels were higher in the past. But damn, when they were, the Earth was a tropical hothouse. But what's bad about rapid CO2 increase is that evolutionary change can't keep up. Species, even entire ecosystems, will get sucker punched. And even so, having less people will help.
I doubt that humans will go extinct, either. We're tough bastards. Maybe there's a population bottleneck coming, but that's often a good thing, in the long term.
https://duckduckgo.com/?q=scientists+letter+warning&atb=v64-...
"Thousands of scientists issue bleak 'second notice' to humanity"
https://www.washingtonpost.com/news/speaking-of-science/wp/2...
"16,000 scientists sign dire warning to humanity - CNN"
https://www.cnn.com/2017/11/14/health/scientists-warn-humani...
"20,000 Scientists Sign "Warning To Humanity" Letter About ..."
https://trofire.com/2018/03/18/20000-scientists-sign-warning...
It's been at least 2 decades since I've met or heard from one who didn't.
Source: http://journals.plos.org/plosone/article?id=10.1371/journal....
I don't agree at all with the half-assed expensive solutions proposed so far.