Short answer, CO2 is bad for the environment. On average, it's also bad for plants at present-day levels.
CO2 is plant junk food, on average plants only benefit from it up to 400ppm (which we've already exceeded), beyond that it begins to harm crop nutrition. Certain plants can benefit from higher CO2 levels at particular growth stages but that doesn't mean those levels are good for most plants most of the time.
And that's without getting into the possibility of the plants being destroyed by a flood or wildfire or drought brought on by global warming that comes along with the higher CO2. CO2 levels that would be ideal for plants growing in a theoretical greenhouse would be too high for everything else in the environment, directly unhealthy for any humans inhaling the air, and catastrophic to established human settlements.