In Sherlock season 4, episode 2: "The Lying Detective", had John not shown up at the exact moment Sherlock was being smothered by Smith, wouldn't he have died? Did he have a backup plan (other than switching the drip bag to saline, which clearly wasn't sufficient)?
It's Sherlock. He doesn't need a backup plan. If he wasn't 100% certain that John would show up at exactly the right moment he would never have gone through with the plan. That's my opinion at least.