I think the "all life on earth dies" scenario is mostly a straw man. I mean, I'm sure there are some people speculating about such things and making comparisons

I think the "all life on earth dies" scenario is mostly a straw man. I mean, I'm sure there are some people speculating about such things and making comparisons with Venus, but so far as I can tell the overwhelming majority of people who are concerned about climate change are worried about much more modestly-bad scenarios. So. Yes, the CO2 level has been much higher in the past. For instance, in the Cambrian period it was much higher than it is now. ... And, in the Cambrian, the surface temperature of the earth was about 7 degrees C higher than it is now. As someone else remarked: no, "the ecosystem" did not survive the Chicxulub event. Life survived it, sure, but it wiped out the dinosaurs (other than the birds). That's kinda a big deal.