Different strokes for different folks. I've got a bunch of LEGO Movie stuff sitting in front of my face nearly all day and I haven't gotten bored of the franchise at all. The story really spoke to me and everything about the movie has been extremely endearing. I'm super excited for the new wave of sets and happy to wait out the years to go until the sequel comes along. That's not at all to say that everyone should feel the same way as I do, but I've gotten the impression that a fair number of people are still interested in the movie, ergo there is a market for the new sets and film and what have you.
Think in terms of Toy Story 3 (I keep using it as an analogy for this movie, I should probably think of something else). It came out years after the first two films, but still drew in an audience and was by virtually any count a success. The trick isn't to keep people's interest over time, but to renew it when the proper time comes. It's a little early to say whether The LEGO Movie will have the cultural permanence that Toy Story has, but I think it is safe to say it has made a large enough impact to ensure there will be people to renew the interest of in 2017 (it was 2017, right? I don't remember).