Surely doing nothing (not actually doing nothing, but rather continuing to burn fossil fuels) leads to, well, ~ infinite warming until we run out of fuel?
Or is the assumption that at 6c we're dead so whatever?
Of course, we don’t know what the feedback loops are, they could kick in and take over. The IPCC is quite conservative about including feedback loops, and mainly deals with direct effects. But the same logic applies either way, within any reasonable range any reduction in CO2 emissions is worth pursuing.
I also personally think the economics for certain low carbon technologies will dominate over a middle/long time period, just that as it stands they won’t take over quick enough to dramatically bring down emissions.
I don’t think anyone in the world can answer the question about how likely these feedback effects are, unfortunately!