Well there is a sequel and we had preview at prison so maybe there is good reason that not a lot of endings, depending exactly what characters in sequel. There was more than enough little clouds dropped in ending to suggest story far from done.
Except even after dating Marc, her feelings for Taylor were to strong to resist even after dating Marc for months and months...how can her feelings for Marc be true love ? Secondly she gives up her education (This is in the ending and no one has managed to explain it away, simply put she again distanced herself from Taylor and no reason for that if she doesn't love Taylor), totally forfeits her cherished freedoms and totally represses her nature and personality. I sorry there is no way she ever going to find happiness and putting a single frame of a wedding does not make a happily ever after life deleting all that happened.
In this they had always been very close, in fact she even all but admits she became a "slut" to try stop thinking of Taylor as a love interest. In start there was even a reference that she had only distanced herself from Taylor recently.
Haley's Story not best comparison as Haley and Nikki totally different personalities and so are MC's.
We really wont know until we see sequel and which characters are in it. Sumo is way better at character development and dynamics between characters which is why the happy ending doesn't sit right (add to it the little dark clouds snuck into ending, the isolation of Taylor with all his social circle lost and the dropping out of college and her isolation from everyone as well). To me this was setting up for sequel not the end of their story. Lets not forget the movie they watched in final either, don't tell me the romance and the dialogue was accident from the two main characters always ending up together, to the boring nature of romance stories and finally ceding there was actually some merit to conventional wisdom. We also have the fact that he was so adamant entire time there was going to be no Nikki ending and no matter the flack or crap he copped on here and patreon he never backed down. This suggests more to me that he had already planned the story into sequel and they needed to be exactly where they are now. I don't think ending was rushed, in fact I think it was very intricate. The superficial happy ending to satisfy the casual players but the shadows looming over future.