See it as an upgrade you can postpone, similar to rome who's Legions are not worth upgrading to longswords until you unlock tercio.I play Aztecs with new version, after a very long time, and I do not really remember the Jaguars' abilities. They have CS10 now, and with Brute force CS13 vs Barbarians. But they loose this promotion on upgrade; as Spearmen they have CS12 and are weaker vs Barbarians than before... Is that intended? If so, I'd prefer there were no unintentional upgrades by ancient ruins.
Btw, I read on Civilopedia that even Barbarian Brutes get the "Brute force" promotion. What do they need it for?