Reasons why Sophitz will sink

Fitz's attitude is pretty toxic towards Sophie. Remember when Sophie didn't want to rush the girlfriend-boyfriend thing? Fitz started calling Sophie his girlfriend around their friends!
Keefe is the best for Sophie. He's caring, willing to listen, and doesn't rush their relationship. Remember the quote, "Until she was ready for more"? Blatant foreshadowing right there.
Keefe makes Sophie smile when she's sad, as shown in Legacy when she says that she's unmatchable. You know what Fitz says? "No one that cares about you wants you to be unmatchable" (I'm paraphrasing)
Keefe risks his life for Sophie, especially in Flashback and Exile. The big fight in Flashback? Keefe ran over to Sophie because he thought she was dead. Fitz didn't do that. And Keefe didn't have to go with Sophie to fix her abilities. Heck, he's been for her since day 1 when she needed to go to the healing center! Fitz only brought her home because he was on an assignment.
Keefe doesn't care about Sophie being unmatchable, while Fitz, clearly, does.
Well, if we want to get technical, Sophitz has already sunken. And Sophie was the one who didn't want to start dating him again.