Views on ancient narratives and miracles change.
From the perspective of modern readers, the fact that Sarah conceived when she was old is interesting but does not seem miraculous.
What seems like a miracle to modern readers is Sarah thinking that her husband sleeping with another woman was a good idea!
(Tonight I took over our church’s Wednesday night Bible study on short notice. They’ve been studying Galatians, and were up to Paul’s figurative interpretation of Genesis comparing Hagar and Sarah and their respective offspring. We needed to recap a bit on the story in Genesis a little as well, which itself led to some interesting discussions, as well as some attempts at humor on my part).