> Almost all countries have ample space that will still be habitable for their entire population in 2100, even if climate change is left unchecked
That's a really dubious one. There's multiple issues with global warming, drought and desertification being one of them (and it probably will not affect a whole country) but the second one is just the max temperature the human body can, withstand especially with high humidity. When this threshold is passed, you can have thousands of people dying at the same time in your country. When every summer, heatwaves take a few of your neighbours you start reconsidering how nice your country is.
Remember, we're going to have more than a 2°C increase in mean temperature by the end of the century, and maybe 4°C. 4°C is the difference between now and the last ice age when the whole Europe was covered by huge glaciers.
Also, many people live near shores, which will be damaged frequently as the see level rises… How would India, who have a borderline genocidal tendency (fantasised mostly at the moment buy still frightening) against Muslims nowadays, react to the massive arrival of Bengali people coming from Bangladesh after a typhoon destroyed their land?
> Italy is a tiny country in terms of land area, so why would they need to take so many?
Because that's where they arrive… I'm not speculating when I talk about Italy, this is happening right now (not 10 millions, but hundred of thousands).
> Anyways, my core point here is (a) we won't need to deal with this migration anyways, because we will solve climate change way before it becomes necessary, and (b) even if we didn't lower our emissions, I'm confident we can solve the migration problem without millions of deaths.
Regarding (a), you should probably read this https://en.m.wikipedia.org/wiki/Emissions_budget
Regarding (b), I admire your confidence, but it sounds delusional in regard of the whole human history.