The abstract question of whether a higher or lower temperature would result in more habitable land (assuming, say, a gradual change over a million years) is completely 100% irrelevant to the issue of climate change.

A fast enough change will:

- Make places where large numbers of people currently live uninhabitable due to temperature and sea levels

- Kill off most plants and animals because the ones in a given place won't be adapted to the new climate, even if theoretically the climate in other places would now be suited to them, in a way that will take millions of years to recover from.

Simply saying "warmer is better" is utterly missing the point.