What if… one of the reasons Sherlock didn’t run away with Moriarty (besides the whole evil mastermind who mindfucked him and murdered people thing) was because he couldn’t leave Joan behind? Just like how he didn’t take the vicodin, he didn’t want to disappoint Joan again, not after M. After Irene’s betrayal, Joan became the one good meaningful connection in his life - she is his salvation and redemption, the friend he (and plenty of others) never knew he could have and his partner. Even though Joan would never see the world as he does, not like Moriarty, Joan is enough to stay for, Joan is enough for him to say no, over and over, to his all his demons.
And maybe Moriarty knew, in a way, how important Joan is to Sherlock, how ‘untouchable’ she is and that’s why she didn’t have Joan killed as a way to ‘free’ Sherlock from any attachments. Because it would have been the easiest way to break Sherlock completely, to utterly destroy him and make him chase her to the ends of the world, but Moriarty doesn’t want that - she wants him to come willingly, to abandon Joan and his goody-two-shoes life and elope with her. She probably wants to take care of Sherlock the same way Joan did, to erase Joan and become the centre of Sherlock’s world again.
To Irene, Sherlock came apart at her death, and it should have been at her mercy that he be put back together. But it wasn’t. Against all odds and what she wanted to believe, Sherlock fixed himself and got sober again, with thanks to Joan. Sherlock didn’t need Irene anymore, he was moving on, and Moriarty really couldn’t have that could she? So she came back into his life like a whirlwind, faking PTSD and being kidnapped to guilt trip Sherlock into running away with her, away from Joan.
They said in interviews this wasn’t going to be your typical ‘two girls fighting over one guy’ thing, but at a deeper level, it kind of is. It’s about two lonely geniuses that found each other and could have been everything together, if not for the criminal ways of Moriarty. She says she’s the only one that sees as he does, but then that’s true vice versa - she’s finally found someone as unique as her, a work of art to her artist heart. Of course she’d ‘fight’ Joan for Sherlock, how could she ever let him go? But in the end, she would always lose to Joan, has already lost long before she ever came back to Sherlock’s life, because Sherlock never loved Moriarty, even if she loves him.
(though how Joanlock feels suddenly took a turn for Moriarty/Holmes I’ll never know).