Have they?
"In this case, their very specific prediction was that warming of between 1.5°C and 4.5°C would accompany a doubling of atmospheric CO₂" https://theconversation.com/40-years-ago-scientists-predicted-climate-change-and-hey-they-were-right-120502
Isn't the problem more that people have been reading that and assuming that it means 3°, not 'possibly 4.5°' ?
That said, the study there seems to assume that the effects are roughly linear, ie. that there are no tipping points.
I hear what you're saying, but I think the real problem is the policy makers, who are without doubt choosing to use the least scary predictions, and pushing even those targets back when they fail to achieve them.