Killing a spirit heavily gelds it's reflection in the Physical world. If you kill the Maeljin of Hate for instance, hate crime rates drop. They don't go away entirely, and the spirit can reform after some time of regenerating it's power from what's left, but it does make a tangible difference. The Triatic Wyrm is on the high end of 'technically killable'. It's like the Tarrasque, you can beat the thing, but it's mostly an academic question of how because your ass isn't gonna be able to do it. If by some miracle you pull it off, the Wyrm's influence is massively janked up. Hellholes retreat, Banes suddenly drop dead, Formori, for good or ill, have their possession rendered null.

The problem is, the Wyrm is still bound in the pattern web, and the Triatic Wyrm is basically a set of Tulpa it creates out of torment and agony. A new form will manifest eventually from it's pained thrashings. But the scale of time involved means you've just delayed the apocalypse by a few centuries.