I'm not really sure if this is tongue-in-cheek, if you're just trolling or if that's a serious argument. I'll try to treat it as a serious argument, just to be sure.
Sure, there are tons and tons of failure modes that are not included in the models. Galactic gamma ray bursts! Rogue black holes throwing the Earth out of orbit! Alien invasion (probably not)!
The question is if they are relevant in the topic of modeling the consequences of continued greenhouse gas emissions into the atmosphere - and the answer is: no.
So, while philosophically your statement is true that "we don't know what will happen, even if the models are sufficiently accurate" - you still get out of bed in the morning and go to work, not wasting any (much?) time thinking about that gamma ray burst coming our way.
> We also don't know what eventually people will do to mitigate or change the climate actively. So again, what will happen is not set in stone, independent of current decarbonization efforts (and their success or failure).
The IPCC has several modes of mitigation that model possible scenarios. That's covered.