As I was writing that statement, I was sure that someone would ask this very same question, but I think explaining it in the middle of that paragraph would have distracted from the overall point I was making, so here it is apart:

Yes, sometimes management needs firing as well, but in the context of my comment above, we were talking about a case where one employee's performance doesn't match up with the expectations of their manager.

To put it in the context of software engineering, if you have an engineer responsible for ten features in a year, and nine of them are coming along just fine, but the tenth was completely bungled, it would be incredibly rare to think that it was time to fire the engineer over the tenth. You're more likely to restructure things so that the engineer has less of a workload, or has help on the tenth, or maybe even just give them a bad review and make it clear that their performance needs to improve.

Firing a manager has an even greater cost for a company and team than firing an employee because the manager is a representative of the relationship between their employees and the company, so firing a manager also damages that relationship for each of their employees.

This is not exclusive to managers. Other roles like engineering team leads and senior architects also have a greater cost, because they're the employees whose roles have a connection to more people than just themselves.