I spend a couple of years at a startup I co-founded; requirements volatility was legitimately very high. We were very experimental, which meant we learned a lot, which meant frequent wild changes. We had a long-term vision, and plenty of theories about the future, but rarely planned more than a couple of weeks out.
This was honestly fine. The product manager (my cofounder) was really good at including us on the necessity of the changes, so it never felt irrational or out of control. Our process was highly tuned for minimum waste. And we were militant about a lot of quality-oriented practices, including unit and acceptance tests, pair programming, paying down technical debt, continuous deployment, frequent releases, and close collaboration with the product manager. That meant that however things changed, we were prepared.
I think if you can do it at a start-up that's shipping every few hours, it definitely works at much lower levels of requirements volatility. To me the real issue at scale is that if agency moves away from the team level and gets concentrated at the executive level, then decisions at least appear irrational, and often are because real-world information becomes ever more removed from the decision makers.
Now, every decision (new or changed) has a cost to be implemented. Some more than others. If you have decision-makers who refuse to recognize that, and think the amount of work they can ask a given staff with a given level of skill to accomplish in a set time period is unbounded -- no process is gonna save you from misery.
If your business leaders are prone to changing their minds at the drop of a hat, you probably aren't going to actually change that. Putting process in place to make it painful to do that is just going to cause pain for the people who have the decision imposed upon them - the CEO's just going to tell other people to figure out how to make it work.
Is there a way to bottom-up build something focused instead on reducing that pain from those pivots? Just-in-time planning whenever possible, for instance, to minimize wasted time invested in would-be future projects?
The timeframe matters, here, though. Are the wild changes once a quarter? Once a week? Once a year?