If you have to put as much energy into splitting the C from the O2 as you got from burning the C in the first place, that means giving back all of the energy that was extracted by the fossil fuel economy for however much carbon you want to convert. That means not just stopping the fossil fuel economy but basically running it backwards enough to undo decades of its output.

Since fossil fuels are being burned as fast as ever and we can't even slow that down (Biden selling new coal leases in the US, etc.), running it at full speed backwards seems like a pipe dream. If we're going to be that utopian, we might as well think bigger and get started on a Dyson sphere.