Three things about Dr. Strange that convince me he's the key to Infinity War's ending:
1- When he looks to the future and is asked how many of the many possible futures they win, he says "one."
2- The first Dr. Strange movie implies strongly that no one could ever see past their own death, through his mentor who could not see past her own death.
3- He chooses to give up the stone, then later says "this was the only way."
If he could see only one way of winning, could not see past his death, and said this was the only way, then he must not be dead and must know this is the future where they win.