theymos, can you provide a reference for that? It's not an intuitive result.

We can't safely do OP_BLOCKNUMBER.  In the event of a block chain reorg after a segmentation, transactions need to be able to get into the chain in a later block.  The OP_BLOCKNUMBER transaction and all its dependants would become invalid.  This wouldn't be fair to later owners of the coins who weren't involved in the time limited transaction.