SEAS isn't the only one going this route. Just look at Disney and their current MM+ plans - while there are some benefits to guests, it really has to do with better separating the money from guest pockets.
It seems to me that in general, while increases in attendance are nice, it's really about how to get more from the customer you already have in front of you that larger theme parks are going for. A bird in the hand is better than two in the bush sort of thing.
But I agree with what said above. In order to charge more, the product's value should be seen as increasing as well. Cuts to operating hours, attractions, shows, concessions, quality, etc. do not convey value in an increased price.
Of course, you can look at this whole issue from the skeptic/pessimist view. The attendance is dropping, they aren't willing/able to spend to correct that, so just say attendance isn't the goal and focus on squeezing out every penny they can from guests who do show up.