I think you missed a factor of 1000 somewhere in there: Each satellite weighs about 1 ton, there are about 10,000 of them. That is 10,000 tons in orbit for the constellation, not 10. Assuming a 5 year decay, that's 10000/5/365 ~= 5 tons / day. Still about 10% of the natural incoming material, but considerably more than your "six hours worth per year".