I do agree that if Draco hadn't fixed the vanishing cabinet Dumbledore wouldn't have died that night but I think that he eventually would have found a way to try and kill Dumbledore. Had it not been that night when there was time for Snape to get there and kill him then Draco might have succeeded and all the plans that Dumbledore and Snape made would not have worked. I do agree that Snape could have stayed double agent longer and that may lead to him not being killed by Voldemort in his attempt to master the wand.

Steph//Puff