This was such a fascinating manga. Someone in this thread compared it to Junji Ito and that feels apt because it all makes a lot more sense if you think of it as a horror manga. And framed as a horror manga, this ending was actually a good one. Riko's feelings and behavior towards Sae were practically a curse and in the end Sae realizes it and breaks the curse. Hopefully she'll move on and find much better people to be around, far, far away from Tomie Jr. over here.