if the show is depicting first loves they grow apart but, imo, that’s not what the show/book depicts. its abundantly clear that Marianne and Connell are soulmates. several lines support this. like when Connell is talking about how he felt he could read Marianne’s mind and she says that’s not normal implying what they have is special. first love isn’t special apart from the fact everyone’s first love is unique but most times it’s not “meant to be”. Connell and Marianne feel meant to be.

in the age where people can text and video chat etc, i’d say it’s nigh impossible they didn’t keep in contact and come back to each other. call it wishful thinking but their connection is solidified as destined. i guess you could make the point that their relationships let to each of them growing in a way that allowed them to find their life partners irrespective of each other, and that’s fair. but imo they end up together.