Focusing on original pair doesn't always have to turn into drama triangle love etc etc, Shiota-sensei to Amai-chan is the perfect example how a story that focused in one pairing can be good as well. The story doesn't have to end after they paired up, there's a lot to show as long as the author can do it but if they can't then the easiest way is kept showing new characters like this manga.
I'm okay with new characters actually if they also kept the focus on the original couple, but the way they appear just as a cameo in this manga I guess that's not happening at all. A lot of people following the manga is because they like the original couple and want to see more of them so their disappointment is understandable.