The ending doesn't need an explaination. Ryo just went insane and thought, burrying Tomie in cement was a good idea. But of course she dies when being cemented. And we all know, that when Tomie dies, her body can literally do anything. Even escaping through a crack. Tomie again succeded to make people crazy and thus the two thought they had a genius plan and wasted their lives.
Well, in my opinion, this chapter gives us the final clue on how to pass on Tomie into the afterlife. Either by burning all Tomies to nothing. Or by making her ugly and die of old age. When the second one is taking place, it would make sense, that this gets past onto the hive mind of the other Tomies. The moment one Tomie becomes ugly and dies of old age, I think every other Tomie will cease from existance, because the only two destinies/requirements are nihilated by that moment: being beautiful and being killed while young.
Though we all know, that this task is impossible to archive.
I personally thought the ending was fine for a series that consists of short stories. The horror aspect weakened in the last chapters though. I wonder if it was intentional?
Btw; I would have liked to see a crazy conclusion story for the teacher Takagi.