If they came out with a 31' model, I imagine many people would buy it just as a display thing.
At that size, it would cost around $250-300. I'm not going to pay that much for a display piece. As good as FFG models are they aren't going to be able to make something that really matches up with a true display piece. For that price I expect a highly detailed model and perhaps even some lighting...
At 31' long you'll have trouble getting it to do much even on a 3x6 table. I don't know the exact ratios of a ISD. But at 31' long, it's going to be what 12-14 wide at the back end. That's going to take up a huge chunk of the board.