I don't see how "carbon will be one of the main limiting factors". Carbon is abundant in our environment -- it's around 0.03% of it and is spread evenly enough

I don't see how "carbon will be one of the main limiting factors". Carbon is abundant in our environment -- it's around 0.03% of it and is spread evenly enough that you can always find it. It's even relatively common in the universe. What is hard to find - and has always been hard to find is cheap energy. And carbon dioxide is essentially the lowest energy state for carbon. It's a dead end as far as energy goes. That's why it's a waste product. If you want to build useful structures then you're still subject to thermodynamics, whether you're using "organisms" to do it or not. So using graphite or any other form of carbon would be a better source of carbon than CO2. The only reason to use CO2 is that it is commonly available (which is why plants use it). And again, no one is proposing removing all the CO2, just enough to keep us in this little ice age we have been enjoying. So there's no reason to worry about running out.