Some comics [0] [1] might be able to get the point across in a way that words seldom do when arguing on the interblags.
You're right to call out that we don't know the "ideal" state of anything. I, for one, am excited about higher mean rainfall, more exotic weather patterns, and the changes in flora and fauna we expect to see as a result. Many species we currently know and love (e.g., basically everything interesting off the west coast of the US or north coast of Australia) will likely be exterminated, but the world will adapt just fine to those new evolutionary pressures. We don't have enough carbon available to create a Venus scenario, so the worst-case probably doesn't kill off all life, and a little chaos can be exciting. I'm sure other people have more sane lists of pros to weigh the cons against, but I try to be honest.
Whatever the "ideal" state is, or whatever positives we can cook up, there are a number of severe negatives which we've demonstrated time and again we have no idea how to address at a global scale, where just winging it doesn't seem prudent:
- Much of the currently populated world will be under water. We have a hard enough time housing a few Syrian refugees and El Salvadorean asylum seekers. Where are we going to put all of Florida, the populated halves of Louisiana, Alabama, the Carolinas, Virigina, ...?
- The frequency and geographic diversity of wet-bulb events will increase substantially. If you're in the wrong place at the wrong time for too long and don't have A/C, you will die. Shade won't save you. Being indoors won't save you. Fans and swamp coolers won't fix it. Beyond the sea level rise, this will render vast swathes of land uninhabitable (like much of India).
- The frequency and intensity of severe weather events -- straight-line winds, tornados, hurricanes, ... -- will increase. We're expecting 20-40% more CAT4/CAT5 hurricanes by the year 2100. In the US, tornado alley will expand eastward. Much more importantly than all of that, the increased energy and humidity will make storms less predictable (harder to evacuate in time, much more devastating).
- The impact isn't limited to people. Our crops have been domesticated over hundreds to thousands of years, and GMOs can only do so much to address hardiness when growing soft balls of sugar and starch on a stem (also applies to corn and wheat, not just typical fruits and vegetables). The last thing you want with a growing population, with some of your best crop land being claimed by the ocean, with neighboring crop land being claimed by oceanic storms many years, with that growing population having less space to exist and encroaching inward toward the rest of your crops, is for crops to be harder to grow. Even if 4 years out of 5 are fine, an extra hot spell followed by an extra humid spell is all it takes to kill your tomatoes, eggplants, peppers, and potatoes (or make a huge dent in their yield that year). Other crops have other undesirable weather patterns. There's a chance this is controversial, but I'll posit that mass starvation (above and beyond the current levels of global starvation) every 5-10 years isn't a good outcome.
And so on. I haven't yet seen anyone try to argue that those negatives are good. Maybe it's a "China hoax" or whatever, but a nicer average climate in Canada (mind you, the extreme weather events will still make most months more unpleasant than the status quo -- averaging weather isn't usually useful) or whatever the proposed upside happens to be won't make up for the cons if the thing is real.
[0] https://xkcd.com/1732/
[1] https://xkcd.com/2500/