As the statheads have pointed out, on field managerial moves don't translate to victories or losses to any great extent. As such their jobs are more fungible than most, and imo most managers are not very smart or very good at their jobs. The same goes for FO personnel. The teams run by good, bad or in between management win roughly half of their games. Whether they deviate from the norm is often significantly influenced by whether their players remain healthy or get injured. So, yes, imo FO is extremely fungible.
Some occupations are more fungible that others based on many factors including the level of skill and training needed to do the job, the pool of eligible qualified individuals, and the impact that the person will have on achieving desired outcomes. Based on these factors, imo, field managers and FO personnel are extremely fungible. There are dozens of people with the same skill sets available for these jobs who would produce similar results.