I think it's not a fair comparison with regards to NS not happening. You're comparing apple and orange here. In Days of Summer, the key takeaway is to see how the main protagonist survive and move on. From the start, you jolly knew that it's going to be a sad story, no matter how you find the scenes involving the main male protagonist and female protagonist (Summer) are cute and lovely. No, no, no. I've seen enough movie to see where their relationship will end. And I was right.
Then again, it's about the main male protagonist's effort to move on, despite his downfall not getting the person he loved the most. We've all seen this coming and there he was, finally able to stand on his feet and move on... The movie concluded nicely with him moving on, applying for new jobs, and meet his new love-of-his-life (autumn) -- which is left to the imagination of the viewers.
So, yeah, comparing Days of Summer with Naruto love subplot, would be an insult to the Days of Summer's author and creative team. They're on a different level.