The IPCC report does discuss what happens if we don't, and it's more than enough to call for some kind of measures. A proper outcome of geoengineering studies will treat that as the control: "This is what we get if we do nothing... this is what we get if we just control carbon output... this is what we get if we apply technique X/Y/Z".
It's just that measuring "this is what we get" is really hard. Temperature is the easiest to predict (and even that is proving aggravatingly difficult on scales smaller than multiple decades), but it's not the only factor. And we need to take ALL of the effects into account to judge what's going to be most cost-effective.
I'm really just asking for somebody to make the case as clearly as possible. A comment downthread told me "Oh, you just throw a bunch of water into the air, and the clouds will fix it." I *know* it's not that simple; it's obvious that a lot isn't being taken into account.
Unfortunately, most conversation about climate change is dominated by the just-plain-stupidity of denialism, rather than getting to ask the hard questions. I want them to be asked, though I'm also sadly fatalistic: denialism has pushed us, as you have said, past the point the ship has sailed. I end up thinking of this as largely academic, and by the time it comes to be implemented it'll be much too late to help. But we're going to do the research anyway. I'm just hoping it will come with enough of the right answers to be compelling to those prepared to understand it.