I think it was really confusing that on MDX it looked like you could make reservations, but none were ever available (it appeared to be always booked full). So, since that's been fixed, I hope that people realize it's walk-up. The listing should really state that instead of just leaving it up in the air, though.
The lack of reservations is also a double-edged sword. I actually like that reservations are not accepted because it's the ONE place I can always eat if I decide to go into the park without three month's notice. But, as you mentioned, that's also why people don't go; they book up all their other meals with ADRs just to be safe.
Unfortunately, I don't think the ADRs or lack thereof is why it's not full all the time. I've eaten there twice and the food and service have both gone downhill. It was awesome the first time I was there. Funny jokes all around, really peppy cast members, quick service, and GREAT food. Last time I was there, we didn't get any jokes except "the monkeys seem to have knocked the food crates off the boat" when explaining why our lunch was taking almost two hours, the cast members seemed to all be off their game, and the food quality was just ok. The slush was awesome as usual, but still no mugs? I'm guessing those are gone for good. It pains me to write this because I've wanted to love this place since it was announced. But, it's just not doing it for me. When I visit next with my family, I'm not sure we'll stop by here (even though they've never been).
Oh their jokes....