With the current trends in the economics of energy, the thermodynamics is not really a limit: the electricity you need is generally available cheaply (or even at negative prices) at certain times of day. The biggest issue is getting anywhere close to that thermodynamic limit, and the cost of building the equipment in the first place. Capturing CO2 from air takes a lot of surface area and a lot of energy before you even do the 'turn into methane' step, and because the energy is only available at certain times, you can't run your plant continuously so the initial costs amortise badly. Those are the areas that need solving to make this concept work.