Latest Osiris lore grab shows that ya killing the Undying Mind has set all infinite forest futures to one where the Traveler dies and the Pyramid takes its place.
Not sure how, but this is apparently what we have to fix.
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It's not guaranteed. The Vex can't process para-causality very well. I'm assuming that the Vex planned to use the Undying Mind in some way to combat/outlive the Darkness. They probably couldn't handle the Mind being killed across the multiverse though, so their predictions got skewered immensely.