The merchandise is a mixture of "Event Exclusive" and your correct if those items aren't sold they'll shouldn't be seen again. I do doubt that they destroy, but we'll never know. There is other merchandise that is being offered that isn't exclusive and will be available via other channels after the event. The WDCC item is an example of something that isn't exclusive and will be seen at DL and WDW if it doesn't sell out. I also thought I saw items that weren't LE, so those are again likely not exclusive.
I was hoping for a real nice Litho, but they don't seem to do those at events these days. I am waiting to see the final edition sizes and prices on some of the items before I decide.