I see this point brought out often. Some counter points:

a) Trees can live for hundreds of years. We're in the midst of transforming how we produce energy and improving energy efficiency all around... A good CO2 'battery' to smooth out the problem can still be very helpful. Think of it as a low-pass filter to help reduce a shock.

b) Forests have a massive number of positive effects beside co2 sequestration, including local cooling and aiding the water cycle. A world with more forests is a healthier world. (See also: riding a bike instead of driving. It saves CO2, and has numerous health benefits.)