Why does France not attack Prussia? A united Germany poses an existential threat to France. Why does France just let the union (which was inevitable at this point) happen without a fight?
I see Prussia going after France regardless. A united Germany not only threatens France, but also Russia. If Germany does not aggressively plant itself as the hedgemon of Europe, then possibly relations don't deteriorate with Britain, lighting a fire under the Tsar's butt to align itself with France. Bismarck would never let it happen. Either France goes after Prussia, or the other way around.
I disagree with you, especially concerning Bismarck. Bismarck was a political genius. He did not want to defeat France for the sake of defeating France. He wanted to use France as a scapegoat to force the south German States into a federal/imperial union under Prussia’s rule.
That’s why he set a trap to have France declare war against Prussia. If Prussia (or the north Germany confederacy) took the initiative of declaring war against France, then not only would the south german States not follow Prussia into war but they would severe the ties that had been forged in 1867 after the Prussian-Austrian war.
These bilateral treaties between Prussia and the south German States were not offensive and defensive but only defensive. And in the main south German States, opponents to stronger integration won the election of the representatives to the common parliament of the Zollverein.
The emerging unified Germany did not need to defeat France to become the number 1 military power in continental Europe. It just needed to flex its industrial muscles as it did OTL before as well as after the unification.
Time and economic development could bring unification with the South German States.
And confrontation between Germany and Russia was not the only possible outcome.