I see the survival instinct as being seperate from logical thought, like one can be aware that suicide is truly what they wish for yet feel held back from ctb because of this irrational instinct. I believe the SI most likely appears when people are in the process of trying to attempt a suicide method or are just about to, causing them to back out.
It's just the way we've evolved, as humans we've evolved to be programmed to survive and I guess the fear of death for some people can be because of the SI, to me it's like the survival instinct exists to prolong suffering.