I would imagine the add-on fee route for extra legroom seats is significantly simpler than creating a new fare category when you consider the number of possible permutations involved. DL now has six fare categories (BE, Main, C+, PS, FC, and D1) with each route having numerous fare filings for each category (although many do not have both FC and PS/D1 fares filed on them). Then, for most Main, C+, PS, FC, and D1 fare filings, you also have distinct refundable fare filings to go with each non-refundable fare filing.