I imagine Hobbes dying a few years after the comics stop. It'll be a slow and painful death too. It'll happen with Calvin just slowly but surely rejecting him, until one day he completely abandons poor Hobbes. Hobbes will have already been tattered by then, being so old and all. Calvin's mom will save Hobbes for a bit, but he might end up going in a yard sale, or in a future spring cleaning of some sort.