The notion of indispensibility on a corporate level is narrow-minded and inaccurate. Utility and value exist on a spectrum.
Since we compensate employees' value with money, why not put it in monetary terms? Is any one dollar indispensable? Is a certain account, maybe a specific asset or group of money indispensable? Maybe in some situations, you would be better off exchanging that for something else. Maybe in other situations you are better off keeping it on the books. Maybe you are trying to grow so you bring on 8 more employees in a certain department, maybe you are looking to scale back or redirect so you don't need 5 more employees here anymore. It's all context-specific, and speaks to no one's purported indispensability, and assumes that everyone is of roughly equal value at their role, otherwise you wouldn't have hired them for that role.
Likewise, if your entire business model is bottle-necked at a single person or made profitable only by delivering one service or by making one big deal with one big client, this is unsustainable in the long term. This is a business that is begging to fail, waiting for some catastrophe to occur and wipe out that account or their star executive assistant to get sick once and wind up on long term disability.
Few can accurately speak to why Mr. Musk fired MB, but he was right to take this time to re-assess, and restructure based on corporate needs, and it should speak that no one should be relying on 'indispensability' to get compensated or be kept at their position.