If it was that easy, we would never have switched from charcoal to coal.

Every exponential has a limit, and in the case of plants the old limit might the pre-industrial or possibly pre-agricultural equilibrium. How much can grow now depends on not just the atmospheric CO2, but also plants and animals we have selectively bread, the canals we’ve built, the the phosphate we’ve mined, the mountain tops we’ve removed to make mining easier, the glaciers we’ve melted, the storms whose frequency we’ve changed, the aquifers we’ve emptied, the lowlands which have become salty from the sea level rises, and almost certainly a lot more besides. (Edit: and if the zooplankton which human activities ultimately encourage produce waste which floats or sinks, the latter being a way to remove carbon from the system, the former puts it back in the air).

Some of that pushes the equilibrium up, some down.