My theory is that Tasha Yar joining the crew of the Enterprise-C and travelling back to the battle of Narendra, when she originally wasn't there, changed a lot of things while others stayed the same.
Except we don't know for a fact if altTasha was there or not. She would have been long dead by the time TNG began, and Sela didn't bother to contact prime Tasha before we first saw her. So there's no way of knowing if altTasha actually existed in the prime timeline prior to YE (predestination paradox), or if we're seeing a changed timeline after YE.
And anyway, I'm not quite sure how Tasha going back in time just to get knocked up and then get killed would have had any influence on wars that the Federation fought with other powers, or where Picard studied.
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