Once the tree slipped from Norn's control, the unity she had proclaimed disappeared. Jin-Gitaxias openly rejected her authority, and Phyrexian factions turned on one another just as Zhalfir's arrival broke Norn's military advantage. Their command structure could no longer disguise competition as perfect obedience.
Norn was reduced from untouchable ruler to a wounded survivor crawling toward escape. Karn chose to remain long enough to confront her. Mirrodin had begun as his creation, and the oil that corrupted it had arrived through his unknowing actions. He refused to leave the final responsibility to someone else.
Karn dismantled Norn and then crossed to Zhalfir. With New Phyrexia phased away and its ruler dead, the command carried by the oil went silent. Remaining invaders became inert or disorganised. The war ended not because every trace of Phyrexia vanished, but because the system connecting those traces to a single controlling will had been severed.
Shella saved Teferi by beheading Vorinclex just as the praetor was about to kill him. Jin-Gitaxias's own creations consumed him when a Zhalfirin strike broke their containment. Karn's dismantling of Norn followed another motive: confronting the ruler of the world he had once created and indirectly allowed to fall. These were distinct failures of Phyrexian command, not one anonymous rout.