I agree with the above, the Sea Cow is in many ways the "flagship" of the line, pun intended, even if it was released later on. The fact that Lego did release it later actually I think was a great one, it gave people time to get aquatinted with the theme and of course see the movie. The concept of the Sea Cow is too far reaching for most people without the context of the Lego movie present in their minds. By that I mean, well, it should be obvious. Look at it, SteamPunk, a Cow? on a ship? I mean, it`s awesome, it`s my favourite TLM set thus far, it`s a beast, but most people would have looked at it and wondered "what the heck is that?" if they hadn`t seen the movie or weren`t familiar with the new line in general.
Most people buy the products they "feel" most aquatinted with, that applies to everything. It`s all about image, the image in the buyer`s mind of the product. Context needs to be provided in order to sell said product, so introducing radically different items to the market (like the Sea Cow) without providing any connection or association to another more familiar feeling product line, would result in people not having that feeling of connection with the product, and thus it wouldn`t sell.
Ok, it is the Retiring Soon thread, so I`ll tie up that point with something more appropriate. A set like this without context in the market isn`t going to sell nearly as well as other more familiar products do. Following that logic, Lego will not retire the set after other Lego Movie sets have been (it will not be the last to go, I`d put money on it). It needs the other sets available to provide the context of the theme, and inspire more people to buy it. It could go before other Lego Movie sets retire, because logically it can`t go after, but my guess is it will hang around until most/all of the other sets are put to rest. Then the Sea Cow will retire along side.
If Lego keeps releasing new Lego Movie sets up until the release of the sequel , that means another couple of years in release, which if you think about isn`t that long ultimately.
So, to summarize, my guess is the Sea Cow will stay available until the Lego stops making Lego Movie sets available, or before the release of the second film, in which case they will want to bring in a new context (the storyline and elements of the 2nd film) to the market. Unless the Sea Cow is featured heavily in the sequel, which I doubt, it will be gone before then.