I haven't gone back to re-read for this, but I believe 10 degrees is presented as a worst-case scenario because if we hit that it's pretty much game over.

The part you may want starts around footnote 20, in a paragraph that notes 4 degrees as a median projection from the IPCC, with up to 8 degrees at the high end of the curve without factoring in permafrost melt and its associated carbon release or any of a variety of other exacerbating factors (footnoted in the article).

There are ways to calculate how much effort or expense is reasonable to attempt to mitigate a risk, I believe that's often part of the job description of actuaries. Part of it involves figuring out what the cost is if the event in question happens, then what the likelihood of the event is.