I like that this didn't end with Saori falling into Mikako's lap, all lovey-dovey. She has some trauma to work through, and them being phone friends (at first) may be a great start. I feel that Saori is attracted to Mikako's youthful idealism and is afraid her dour fatalism will snuff out that spark.
Mikako needs to be a bit more cautious, while Saori needs to be a bit more open. THey both have lessons to learn and starting out SLOWLY looks to be the best course for a future with these two.
I love stories about a younger woman opening an older woman's heart. But there is no rule that it has to be easy or quick.