The problem is that not all plants will win there, and probably not the plants we rely on: for the higher CO2 levels to be useful to a plant, it needs to be growth-bound by carbon acquisition.

Guess what no fertiliser I've ever heard of contains? Carbon. Carbon and oxygen are the two things agriculture has no need whatsoever for. If anything, higher carbon level might make agricultural products worse by making the plants grow faster at the cost of nutritional content.