According to Elon Musk, there's "an argument" stating that the current level of carbon in the atmosphere (more than 400 ppm) is better than it was some hundreds of years ago (200 ppm). He even calls these low carbon levels carbon starvation.
https://www.youtube.com/watch?v=kzlUyrccbos&t=45m4s
What is this argument based on? What harm would it do if the level fell again to 200 ppm?
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.
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 ubiquitous.
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.
Musk didn't state his reasoning, but the only benefits of higher CO2 levels that I can see are that a) plants grow more vigorously, so therefore we can expect better crops, and where damaged forest is left alone it will regenerate more quickly, b) climate warming is an insurance against the next ice age or little ice age. There was a Little Ice Age between about 1350 and AD 1810 (estimated dates vary, as there were no accurate climate records in the Middle Ages). LIAs are believed to be associated with periods of low sunspot activity called Maunder Minimums, and some scientists say we are now on the verge of another.
Milankovitch Cycles only tell you when a proper ice age is likely, they are rather complex and don't accurately predict Ice Ages. We can be sure that there will be another ice age, but no one knows exactly when.
The down sides of higher CO2 levels are so well known that it shouldn't be necessary for me to enumerate them. The question was: are there any advantages to higher carbon levels, so I have answered it.
