Why can't a transporter save a copy in the buffer and regenerate a copy at that instant if the original doesn't return? There have been many references to patterns degrading quickly and the transporter needing the exact matter it started with however there are examples where these rules have been broken. In TNG "Relics" Scotty kept himself locked in a buffer for 75 years. In TNG "Second Chances" a copy of Will Riker was created due to an energy surge at the time of transport. The amount of energy required should be related to the composition of the human and surely there is enough stored in the ship or they could convert an few hundred tons of rocks and shrubs to regenerate essential crew such as the Captain if he happened to be killed. If they have a corpse even better most of the matter is there just some added, removed or rearranged.