This sounds odd, Whats the rest of the story. A boomer CO does not get relieved unless he has a real issue. You don’t (or at least did not) become a CO without being vetted pretty well before hand. Well I guess we are in a new age where vetting is a thing of the past heck our prez-ele was never vetted.
There has to be more to the story. And the captain’s career is over. What happened?
That is not true at all. You have made it through training and subordinate positions, but command at sea is a separate calling to itself. Some folks cannot handle the pressure. Some micromanage and cannot trust their people. Some get concerned about the small stuff and cannot set priorities, which for a CO are a fit and trained crew and a well maintained ship and a focus on mission effectiveness. Others turn out to be slack and don't maintain operation discipline, or may care more about self promotion than the welfare of crew, ship and mission.
Some of the folks who do best in subordinate positions are totally unfit for command.