Since you can do the math, surely you realise that you are misrepresenting what is at issue. The quantity of CO2 matters not because the atmosphere is full, and another 0.000001 percent contribution by CO2 will make the cup overflow. The quantity matters because CO2, and other climate-changing substances, are part of a system in dynamic equilibrium. More CO2 and heat-trapping substances in the atmosphere means more energy retained in the atmosphere, oceans, and lands. That in turn changes energy distribution flows between them. That it turn changes climate. That in turn leads to bouts of extreme weather: heat waves, cold spells, storms, floods, all of magnitudes and frequencies greater than before.
Also, since you can do math, you are aware that 400 ppm is 2.67 times 150 ppm. Whether that is “near” or not depends on how many ppm the amount varies over time spans that matter. Since it has taken centuries for this concentration to get from 200ppm to 350, and decades to get from 350 to 400, the trend for acceleration away from 150 ppm. Thus we are not “near” it.
By “plants” you probably meant more than just trees, corn, and grass; but I should clarify for everyone’s sakes that only half or less of the atmosphere’s oxygen is generated by land plants. 50%+ is generated by phytoplankton in oceans and lakes. So compared to a land plant die-off due to too little CO2, a wather plant die-off from temperature change or pollution is a bigger threat to human survival.