That 2004 season really was quite a special case though. And, if I really want to split hairs, if you take out the two Bradford crowds from that the average is only 15,161. Take out the final game vs London, which was when the LLS was presented and got a definitely above-normal 17,329, and it's 14,965.
Should Leeds have grown their crowds since then with a poorer-performing team and despite the loss of its biggest game every year? I continue to think the club doesn't do enough on the crowd and marketing front and has probably been happy for some years with very high corporate revenues, together with high yields and lower averages for regular fans. That was an approach which got them through the successful years and helped pay for a large chunk of the stadium redevelopment. The approach has actually changed a bit this year with season ticket prices somewhat reduced.
However the club definitely has a higher baseline level of in-person support now than it did 20 or 30 years ago.