Most integration playbooks treat culture as a communications problem — a town hall, a values deck, a joint offsite. Culture friction in a merger is usually a decision-rights problem wearing a culture costume: two teams that made decisions differently before the merger are now colliding over whose process wins, and nobody has actually decided.
The integrations that hold are the ones that resolve the decision-rights question explicitly and early, even when the answer is unpopular with one side, rather than leaving it to informally resolve itself over the following two years of friction.