My solution may be an over simplification but here it is. It's my belief that most TB's disappear because some cachers don't know what they are. For instance, somebody signs up on geocaching.com (not a premium member), finds 8 or 10 caches along with a TB or two and then quits the game. I personally never put a TB into a cache unless it's a premium member cache. This way only paying members will have access to the TB. If everyone would do that, I'm sure the number of missing TB would drop.