"At first reading, I agree with most of what you have written here. Warmer is better; CO2 is good for plants, and current levels of the much-maligned trace atmospheric gas are certainly no danger to man or most other living things, at least AFAIK. In any event, CO2 levels are (comparatively) quite high in buildings, submarines and other enclosed spaces where air-breathing creatures are present. From what I understand, humans routinely work in such spaces where CO2 levels approach 5000ppm. "

Yes, but as far as I know, there is no benefit for humans to be in areas in which there are CO2 levels approaching 5000 ppm, whereas high CO2 levels are a benefit for growing crops.

Suppose you were to compare urban setting in which CO2 levels were 500 ppm to those with 2000 ppm. In an Urban environment with 2000 ppm, one would be required to refresh indoor air at a faster rate than to a urban setting with base level being 500 ppm. This will mean higher heating and cooling costs- and/or situations in which poorly ventilated areas are getting CO2 levels higher than 5000 ppm.