> There was a time when investors/the board could fire a CEO making terrible decisions

This is still by far the most common case, and in fact it is probably becoming more common because large index funds have recently increased requirements related to ownership share structuring (https://corpgov.law.harvard.edu/2017/08/05/sp-and-ftse-russe...).

In this specific case though, the investors knew that Zuck had complete control and they decided to invest regardless, so I do not have any sympathy for their complaints.