Why would this approach be smarter than killing the younglings?

That wouldn’t have worked. Those younglings had the potential to become Jedi, who could have rose up against the empire.

That is not an option. By the end of Revenge of the Sith, the Jedi Order has essentially been eliminated. The infrastructure for training Jedi is gone. Put more thought into this thought experiment. You might start at figuring out why this would have been better for Anakin. 😉

Non Sith force users are ALWAYS considered a threat to the sith. There isn’t exactly a benefit to keeping them alive either, unless, like the third sister, they were trained into inquisitors.