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.
The IPCC does speak towards SRM etc. but I think large scale geoengineering isn't really part of the proper menu there. So no, the IPCC isn't all covering and all conclusive. EDIT: in the sense that it treats certain things as high risk and rather theoretical - which is true, but doesn't mean they should be off the menu.
If we cannot even agree on simply cutting fossil fuels, how on earth can we ever agree on large scale geoengineering projects that will have massive negative consequences for many countries involved?
Further climatic disruptions will make the already dire situations in many countries untenable. That situations like that do not tend to improve by unilateral actions of their neighbors if the threaten to make a barely acceptable situation even worse.
https://www.thetimes.co.uk/article/the-first-water-war-is-un...
The worst part is, once the killing starts, people will rationalise it to themselves, saying "We didn't have a choice".
I guess I'm in a sarcastic mood today. Yes, of course that will cause mayhem. It's one of the reasons I see climate change as such a threat to our all civilization. We're fearsome tribal creatures and the coming waves of desperate peoples will almost certainly play into the hands of nationalist fearmongers and firebrands.
I wonder if we will see some kind of -literally - ecofashism rise from that.
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).