Trees with their own conversion capacity are indeed a good interim solution, but in the long term this form of carbon storage isn't as permanent as rocks, especially given how prone trees are to dying and decaying in the biosphere of today. The Devonian didn't have that problem. For example, parts of the Amazon have become greenhouse gas emitters even when human-caused deforestation is discounted.
There's also the very real possibility of forest fires releasing huge amounts of carbon at once. Even dead trees preserved in peat bogs could burn and release carbon in this manner.
Rocks have their own weaknesses such as ocean acidification, but relatively speaking they're much more stable.
Overall, we are actually very terrible at understanding global consequences. Earth as a dynamic system is just too big and interdependent. Yet Earth's biosphere has recovered from cataclysm after cataclysm. Usually at the expense of 90%+ extinction. A big reason the carboniferous period happened is because fungi had not yet developed the capacity to digest cellulose. We're in a different regime now, and there is no telling what will happen. But one thing I think is pretty clear: we aren't going to tech our way out of this and make up some magic carbon sink. So the Earth is probably headed for a really hot period with lots of carbon in the cycle, until it stumbles on a new way to sink it. Either that, or it will head towards a semi-permanent desert planet.
There's also the very real possibility of forest fires releasing huge amounts of carbon at once. Even dead trees preserved in peat bogs could burn and release carbon in this manner.
Rocks have their own weaknesses such as ocean acidification, but relatively speaking they're much more stable.