Planting 10 million saplings themselves is not wrong, but the areas that will be selected for afforestation typically and invariably suffer from drought like conditions and face the problems of soil erosion. And if no measures are taken beforehand, the saplings will soon wither away in the sun or get washed away in the run off. There was a joke around my school that we used to plant the sapling at the exact same spot every year.
Yes, the trees are very effective in stopping soil erosion but only after they are strong enough and have taken root.
You can check videos on afforestation efforts at Sadhana/Auroville Forest for more details.
Which is why I mentioned planing them just before monsoon.