The only slim reasoning for Daniel living is this: he is the sole sufferer of memory loss as an effect of temporal displacement. Everyone else suffering from temporal displacement sees their consciousness flit between different times in their life. His memory loss, therefore, is possibly a result of something else:
The only other significant mention of memory loss has come from Richard as he agreed to take Ben, i.e. “He will remember none of this.” If Daniel, mortally wounded but not yet dead, is quickly taken to the Temple, what exactly would he forget?
Implied memory loss occurred after the Discharge. After the whole sky turns purple, Charlie casually plays with Aaron and tells Claire that “nothing happened” when she questions him. His tone never struck me as false; could it be that an effect of a discharge of the Swan is memory loss? If Daniel lives to see the Incident, does it cause his memory loss?
However, the implications of either of those possibilities get iffy. If Daniel lives to suffer memory loss from either the Temple or the Swan, does he then return to a later date (in the 1990s?) suddenly exhibiting that memory loss? And if he does, what happens to the other him who is researching at Oxford at that point in time?
I think it’s most likely that the reason that only he suffered memory loss may go unexplained, but it will be revealed that, like Desmond or Teresa, Daniel’s consciousness has been flitting through time ever since he acted as his own test subject. Due to the memory loss, he does not realize how his consciousness moves. He is instead left with only vague impressions that effect him for reasons he cannot grasp; he cries at the wreckage of the plane because his mind just recently was on the island with the survivors of that crash, and he simply can't recall things he experienced in that other segment of his own timeline.
Daniel’s dying words seemed to resonate with Hawking. Her expression was not one of disbelief, but one of fear. She may currently, in 1977, be pregnant, or have already given birth to a young Daniel. Clearly an older Hawking is well aware the the man she shot in 1977 was her son, but it seems that Hawking may realize this as soon as it has happened.
I believe Desmond is the titular variable. Hawking expressed some certainty that he would die (going so far as calling him a causality) immediately before revealing that she used to know what would happen, but no longer does. We've seen more than one situation with Desmond's involvement that did appear to change:
Daniel arrives in the past and entreaties Desmond to find Eloise. Desmond quite suddenly awakes in present day with a memory of this event. The question is whether he simply could not recall that meeting, or if that memory was new because that meeting had not actually happened yet.
When Desmond's consciousness was wavering between his past and his present, he performed a lot of actions that clearly were new events for him. He had never in his known past met Daniel. He had not asked Penny to call a certain phone number on Christmas. While occurring in his past, i.e. a time period he had already experienced and remembered, those events were still new to him.
Therefore, if Desmond is able to return to the DHARMA era of the island, his actions would in fact be capable of changing the future.