The bigger problem is that it's an absurd amount of trees.
Estimated cost is a few hundred billion dollars. A lot cheaper than most methods of carbon sequestration. It also has the benefit that it could restore ecosystems if done well. People who focus just on carbon emissions miss that the are more systemic problems with destroying ecosystems i.e. in the long term biosphere collapse.
The larger problem is that it takes 50 years (or longer) for most of the carbon sequestration to happen.