Sentient carbon units organized in civilizations may be the rarest of all, but they may be able to create other, non-carbon, life forms that will survive them.

And eventually, these life forms may stumble on their Creators, or on others like them. Sort of like V’Ger.

There are examples of single cell carbon units flooding atmosphere with highly corrosive oxygen, enough to precipitate iron from ocean water over millions of years. Very complex carbon units can also change the atmosphere but at accelerated rates.

Furthermore, a planet covered in carbon units makes it very difficult to map geology and resources.