The Shortest Meta I will ever write:
At the end of TAB, Sherlock talks to John about what he thinks their life would look like in the future. He goes to the window and he makes his Victorian mind palace change to present-day mind palace. We never see him leave the mind palace because all of series 4 is that projection, exactly as he said it would be. That’s why The Final Problem features John’s Victorian chair and flashes of The Waterfall scene.
The reason “TFP as John’s TAB” is an excellent reading with only a few minor holes is because Sherlock is trying to see himself through John’s eyes – he did that multiple times in TAB. The times we believe we’re getting John’s POV in series 4 is actually Sherlock taking it upon himself to attempt to empathize. That’s why they can both see Mary in TLD. That’s why the greenhouse scene concerning Irene happens twice – once through Sherlock’s POV, once through John’s. Sherlock and John have so much in common, this isn’t a new idea at all. They were mistaken for one another in TBB. They are two halves of the same whole. Series 4 doesn’t switch POVs, it’s Sherlock controlling both.
“I’m you, aren’t I?”