We know that Red matter makes a black hole portal you can travel through. We know that how far back you travel depends on how soon after the initial formation you enter. We know that a formed portal deposits you in the same timeline as previous entrants. Therefore, changes made by the first travelers don't effect the origin timeline because that would prevent later travelers from entering at all. If Red matter worked like, say, the Enterprise C rift, then Old Spock would never have had a chance to enter at all, because the Prime timeline would have been overwritten before he had a chance to enter.