The quality of the leather is irrelevant to the argument. We only care about how much each method costs.

Like (E), we already know the overall costs outside of waste disposal are roughly even for both methods, so any discussion of the costs of other parts of the process is irrelevant. Even if some aspect of tanning is more expensive for one method or the other, the costs will always be roughly equal when ignoring waste removal.

(C) provides a potential reason why the costs outside of waste removal are similar for the two methods, but we don’t need this information. It could be because of new tech, cheaper materials, or government subsidies without affecting the argument.

If (D) is not true—if disposing of biological waste costs way more than normal waste—then it doesn’t matter that less waste is produced. Biological waste removal would cost more overall, and the biological process would thus be more expensive.

Like (B), we already know that the overall costs outside of waste disposal are roughly even for both methods, so any discussion of the costs of other parts of the process is irrelevant. Even if some aspect of tanning is more expensive for one method or the other, the costs will always be roughly equal when ignoring waste removal.