If plants were growing poorly when atmospheric CO2 was 200ppm, it was probably because Earth was in the middle of an ice age and covered with glaciers, not because plants were starved of carbon dioxide.
All things being equal, increasing the amount of carbon dioxide available in the atmosphere can benefit some plants, but the problem is that all things do not remain equal.
- CO2 is only a fertilizer in environments where CO2 is the limiting input for a plant; plants in the outdoors may receive too little moisture, too much warmth, or too little sun for additional CO2 to make much difference.
- Adding CO2 to the atmosphere also changes global and regional climates, reducing precipitation and increasing temperatures in ways that offset benefits from CO2 fertilization.
The authors of Plant growth enhancement by elevated CO2 eliminated by joint water and nitrogen limitation in Nature Geoscience found that
the presence of a CO2 fertilization effect depends on the amount of
available nitrogen and water. Specifically, elevated CO2 levels led to
an increase in plant biomass of more than 33% when summer rainfall,
nitrogen supply, or both were at the higher levels (ambient for
rainfall and elevated for soil nitrogen). But elevated CO2
concentrations did not increase plant biomass when both rainfall and
nitrogen were at their lower level. We conclude that given widespread,
simultaneous limitation by water and nutrients, large stimulation of
biomass by rising atmospheric CO2 concentrations may not be
Scientific American tackled the question in Ask the Experts: Does Rising CO2 Benefit Plants?, concluding that "Climate change’s negative effects on plants will likely outweigh any gains from elevated atmospheric carbon dioxide levels."
Researchers cited in that article pointed out that added CO2 provides diminishing returns, with Samuel Myers, principal research scientist in environmental health at Harvard University, noting that “We know unequivocally that when you grow food at elevated CO2 levels in fields, it becomes less nutritious."
The second part of your question was what would happen if CO2 levels fell to 200 ppm. This is unlikely to happen on human timescales; the current CO2 level is about 410ppm, rising 3-5 ppm per year. CO2 levels were at about 280ppm for most of human civilization, only increasing substantially when fossil fuel use became widespread in the 1800s. CO2 has a residence time in the atmosphere of several centuries.