Why is it good to reduce Earth's temperature?
Discussion
Because without the SO2 things start to look like this 
Why is that bad?
Geological history shows temperatures many degrees Celsius higher than at present (eg Hippo bones in the Thames)
Because billions of people weren’t living on earth, especially coastlines during other geologic eras. It’s not bad for the planet. It’s bad for our species
You're considering costs and not benefits
Warmer temperatures would free up vast tracts of land for cultivation in Russia and Canada, as well as reduce weather-related death, as well as enhance plant productivity via greater rain & CO2
Pretty sure more ≠ better with plants. More C02 and water likely isn't better for plants, just like water and oxygen being good for humans, but excess of either is detrimental.
No, infinite CO2 would not be good
But current atmosphere is 400ish parts per million ie 0.04%
Greenhouses pump in CO2 to 3x that or more - because it's good for the plants