I’ll start with the usual disclaimer that I don’t entirely care why/how Sherlock pulled off the body swap. It’s enough for me that it happened.
But you’re all getting bored of that answer, right? Okay then—let’s start inching our way to the speculative end of the pool.
We know that Sherlock couldn’t hang around and play dead forever. Eventually he was going to have to switch with a body double or a corpse, or just take off and leave Molly to handle things.
If Sherlock was going to leave or do a switch, the safest place to do it would absolutely have been inside the hospital. Stay on the trolley, let the Homeless Network operatives push it inside an elevator, and do the switch there. No way John or the sniper could see that.
Why didn’t Sherlock do that? Why would he take the risk of being seen doing a body swap on the street?
Because he didn’t want to be in the hospital just then. He wanted to be somewhere else.
It’s easy to see why Sherlock wouldn’t have wanted to be in St. Bart’s—nothing for him to do there, really. He didn’t need to stand around watching Molly sign his death certificate.
So where would Sherlock have wanted to be instead?
Technically, it could be anywhere. We don’t know what his plans were. Maybe he decided it was time for a vacation and ran off to the beach. But if you think he went just anywhere—that he was trying to escape the scene of the crime—we’re back to the question of why he’d do the body swap on the street. Once again, it would’ve been easier and safer to let himself get taken inside, calmly collect his beach bag, and leave from there.
I suspect that Sherlock did the body swap on the street because he wanted to stay on the street. Two reasons for this immediately come to mind.
The first is that staying on the scene was the best way to make sure his trick had been successful enough. That John believed he was dead and, more importantly, that John didn’t get shot.
The second is that Sherlock is really much more inclined to take action than to hide out. (He came the closest to being passive and hiding that we’ve seen him earlier in the Reichenbach story, but he realized that didn’t work out well and went back to his usual self in a big way before the ending.) As long as Sherlock was alive and well after the fall, he could’ve taken that opportunity to do something useful like track the sniper while the guy would least be expecting it.
And because I can only go so long without dragging canon into it, here’s a passage from The Adventure of the Empty House in which Holmes is explaining what happened after his “death”:
But I struggled upwards, and at last I reached a ledge several feet deep and covered with soft green moss, where I could lie unseen in the most perfect comfort. There I was stretched when you, my dear Watson, and all your following were investigating in the most sympathetic and inefficient manner the circumstances of my death.
At last, when you had all formed your inevitable and totally erroneous conclusions, you departed for the hotel and I was left alone. I had imagined that I had reached the end of my adventures, but a very unexpected occurrence showed me that there were surprises still in store for me. A huge rock, falling from above, boomed past me, struck the path, and bounded over into the chasm. For an instant I thought that it was an accident; but a moment later, looking up, I saw a man’s head against the darkening sky, and another stone struck the very ledge upon which I was stretched, within a foot of my head. Of course, the meaning of this was obvious. Moriarty had not been alone. A confederate—and even that one glance had told me how dangerous a man that confederate was—had kept guard while the Professor had attacked me.
So basically, in the original story, Holmes:
- Stayed at the scene of his “death,” but found some cozy little place to hide.
- Watched Watson react to his “death.”
- Then got tangled up with the henchman of Moriarty’s who had been assigned to watch the whole thing.
Feel free to compare those points with the speculation above, and draw your own conclusions.