I learned in high school bio (40 years ago!) that only 0.02% of our O2 comes from land based vegetation, and that is because Phytoplankton constitutes 99.8% of the vegetation on Earth.
As for the "miracle drugs from plants" idea, most of the plants of the jungles were well documented and analyzed before the close of the 19th century. It's a false argument for many reasons, but the best reason is that their medicinal properties are known and proven, but their use is forbidden by law at the demand of the AMA and the pharmaceutical industry.