<p>I didn't say the Kage wouldn't be prepared to make more junchuriki, or that they didn't make plans period; just that they can't force that to happen purely for the sake of plans.
</p><p>The reasoning of keeping around powerful forces purely to meet threats that may/may not exist... is not legitimate on it's own merits, even for good leaders imo. No one knew about Momoshiki or Kinshiki; if they had, then they'd have made sure to take measures to prepare for them - but without that knowledge, there was no reason to build up their forces. That is also not a legitimate reason to force Kurama to be passed down the family lines... if for no other reason, he has a say in the matter if he ever wants to be free of being confined to a human host.
</p><p>^The same applies to Boruto and Himawari; even after they meet Kurama, what if they don't want to be jinchurki? What if Kurama doesn't want to be confined inside them either, even with his newer views, but would rather choose someone else?
</p><p>The general point is, preparation and plans only goes so far, without a credible reason or threat to direct them toward. They'll still fight and make every effort to oppose enemies, but they won't be needlessly paranoid or overly cautious anymore - not as they were in the past; that's the entire point.
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