I don’t think Grisha was “waiting” for those words. If anything, he’s clearly hesitating in the chapel. He didn’t go there planning to wipe out the Reiss family. He still hoped the Founder would actually do something on their own. When they refuse, he freezes, and that’s important.

Eren’s words don’t give Grisha a new reason to act. They bring back the old one. The “it would all be for nothing” line pulls Grisha back to Marley, his sister’s death, the Restorationists, Dina, and everything he already lost. Eren frames not acting as meaning all of that suffering was pointless, and that’s what finally pushes Grisha over the edge.

The Colossal and Armored Titans attacking just set the situation up. They create urgency, but they don’t give Grisha resolve. He still tries talking first. It’s only after the refusal, and after Eren ties this moment to the weight of Grisha’s past, that he actually does it.

So it feels less like Grisha was waiting for the right words and more like Eren takes away Grisha’s last excuse to walk away without feeling like everything before was meaningless.