Not all dramas are just there to prolong the plot. This was something set up from the start, as both a motivation for Nagisa to even look in the direction of Mashiro, as well as later an emotional bomb those two dorks needed to move forward with their relationship. Meanwhile, while we moved between those two points, we gained new plotlines, motivations, characters, and all sorts of goodies we can explore.
Of course, I can't say for how long this series will go on and if it'll explore more of those two, but it served as a hell of a prologue for the entire series as well as a good start for their relationship, in which I hope we will follow those two (and everyone around them of course) for a bit longer.
Now that I got that out of my system, I'll say it again. Those two are such dorks. And I love 'em for it.