Frame challenge: a reset to normal would make the Earth most habitable.
The question asserts that future humans:
aren't overly worried about further damage to the biosphere (because it's already been thoroughly wrecked)
However, 80 years is not nearly enough time to destroy even 10% of global species. According to the WWF, "between 0.01 and 0.1% of all species will become extinct each year." If we assume a worst-case-scenario of 0.1% extinction per year, we'll still have 92% of today's species by 2100. That's terrible, but still minimal enough to make saving what's left worthwhile.
After all, further damage to the biosphere would directly cause damage to humans.
* Loss of biodiversity threatens the very agriculture that increasing temperatures is intended to promote, because diversity keeps soils productive.
* Biodiversity ensures that adequate nutrients are available in different geographic regions, so killing more species might feed wealthy agrarian countries, but it would starve less developed ones.
* Increasing temperature changes the movement of disease vectors. As climate change progresses, we are seeing a plethora of new diseases (and old diseases in newly-warm places) even today.
Not to mention the other physical effects of injecting more energy into the Earth's atmosphere / hydrosphere:
* More tropical storms
* More floods
* More droughts
* More forest fires
These events are not only deadly, but expensive. 876
They would threaten humanity as it expanded; less efficient soils, scarcer nutrients, worse weather, and worse diseases would combine to yield no net benefit.
Furthermore, the assertion that increasing temperatures would yield more productive land is inherently flawed. A warmer Earth may mean hospitable poles - but it would also mean far less land near coasts, as well as inhospitably hot land near the equator. I will concede that the poles are warming faster than the equator, so you might see a minimal net gain in arable land, but sea level rise may reverse even that. You would be better off engineering plants to live in salty soils or hard permafrost than to disturb ecosystems that have been stable for millions of years.
The bottom line is, we have largely only seen negative environmental impacts since the beginning of human-driven climate change. If humans are offered the technology to fix the Earth's climate in 2100 - at which point the ecosystem will still be salvageable - they will begin the process of rebuilding instead of worsening the damage.
TL;DR The optimum temperature of Earth for humans is the one we evolved with, because we are dependent on the ecosystems around us. 591