Each chapter is required to send (to earth) back samples of their geneseed for testing and storage. They don't just have what the chapter itself has on hand. Consider it a 'reserve'.
Should the chapter BE wiped out, it is possible for the High Lords of Terra (HLOT) to order the creation of a new chapter using the stored geneseed of the now destroyed one. This does require a couple of centuries minimum (to create enough zygotes to implant into hosts to start the procedures) and a large outlay in hardware to outfit the new chapter (should any chapter assets still be available, they will of course be utilised).
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