The real success of Heartland was and still is the Amy and Ty love story. The dating in season 17 especially Amy's relationship with Nathan is simply the writers attempt to keep the storyline interesting, and not boring. I believe if Ty comes back to the show Heartland will continue with Ty, Amy and Lindy together again. If he doesn't return the writers will conclude the series with Amy unable to accept another(not just because Ty can't be replaced} because of her continuing strong love and feelings for Ty. She will conclude that her peace and happiness is with her as a single mother raising Lindy in the manner Ty would have wanted. In my opinion, because we had 14 years of this remarkable love story, whereby the viewers got to intimately know Ty, it makes it nearly impossible to erase him from the viewers minds and on a larger scale future Heartland seasons.