I feel like even though Shou was chasing after Shimizu the whole oneshot there were subtle hints that Shou and Saya would end up together. Like the way Saya looks similar to Shimizu (especially after her style change that Shou comments on), Saya supporting Shimizu in studying when no one else would, and Saya once again being there for her after Shou's heart was broken for months. Not to mention Saya saying "something domestic" which is probably referring to the setting of Tsurezure Biyori, I'd like to imagine that's where they visited and ended up wanting to move there when they were older and fulfilled that.
Besides that, I think this oneshot is a nice way to show that basing your whole future on a person you barely know is a horrible way to navigate life, but a lesson/mistake that most young people will learn through firsthand experience. I'm happy it ended up working out for both of them in Tsurezure Biyori