Earlier quoted context omitted.
You say that people who never read the Bible will never find out about Numbers 31, yet here you are telling many people who never read the Bible about Numbers 31. I almost wonder if the average atheist is more likely to be aware of this Biblical narrative than the average Christian, but I'm not sure what that would prove if true. Anyway, it's worth linking to the Wikipedia article which provides the background to tha…
I'm not seeing where in the link you shared it states that this was the standard practice. But that aside, if we're allowed to dismiss certain aspects of the bible because they are no longer culturally relevant, how do we decide which tenets should be followed at all?
To give just a couple of examples, there is the case of Pharaoh ordering the killing of all male children at the start of Exodus[0], and in Esther chapter 3 we see that the antagonist Haman's plan was "to destroy all the Jews who were throughout the whole kingdom of Ahasuerus".[1]
Anyway, I don't think Christians should dismiss aspects of the Bible that are culturally difficult, but there are principles for applying its contents to modern situations. The relevant principle here would be that the Ten Commandments are to be generally followed at all times, but God may choose to give specific instructions for specific situations which override those commandments. Reassuringly, it wasn't up to Moses to decide whether or not to attack the Midianites, as he was given that as a direct instruction, in his capacity as the political leader of the Israelites.
[0] https://www.biblegateway.com/passage/?search=Exodus%201&vers...
[1] https://www.biblegateway.com/passage/?search=Esther%203&vers...