(This is one of a series of Software Engineering Maxims Which May or May Not Be True, developed over the last few years of working at Google. Your mileage may vary. Use only as directed. Past performance is not a predictor of future results. Etc.)
Project plans change. It happens.
When plans change too often, or when a crucial plan is suddenly cancelled and destaffed, we burn more than just the time which was spent on the old plan. We burn confidence in the next plan. People don’t commit as readily and don’t put their best effort into it until they’re pretty sure the new plan will stick.
In the worst case, this becomes self-reinforcing. New plans fail because of the lack of effort engendered by the failure of previous plans.