In fact, the outcomes of climate change become "unacceptable" long before reaching anything like a worst case, and that should be the basis of climate change decision-making. It's not easy, since conclusions regarding "unacceptable" climate change are likely to vary substantially by geography, by business sector, and by time frame. And any consideration of "acceptable" climate change has to address climate change as a threat multiplier, and the potential for climate change to destabilize economic and political systems (systemic climate risk).
Arguably, given how adapted humans are to the last 10,000 years of global climatic stability, there may be no globally "acceptable" level of climate change. Having a serious conversation regarding "acceptable" vs. "unacceptable" climate change, however, requires realistic consideration of the many uncertainties surrounding climate change. The more uncertainty, the more risk. The more risk, the less likely it is to be acceptable. This challenges the dominant approach to thinking about climate change today, namely "expected" climate change