Switching the main love interest seems pretty unlikely, but Shouko certainly could be it, should it happen. However, Shouko doesn't necessarily even see Nagai as romantically interesting. She definitely does see him as interesting, yes, but that doesn't mean she would like to date Nagai. She does understand Nagai better than most other, certainly better than Aizawa. I suspect Aizawa loves her own image of Nagai, not the real Nagai. Shouko, however, can see Nagai for what he is and could thus handle him just as well as Hinohara can, or better since she hasn't got the teacher's inhibition Hinohara has, like I said earlier.