Nothing should be indispensable as it’s loss or failure can then be disastrous. If, on the other hand, every system has a back up, it can continue to function. If we give all of our land over to one crop (a monoculture) and it fails, we starve, but if we grow as wide and diverse a range of edible plants as possible (a polyculture), we still get to eat if some of them don’t make it to harvest. Similarly in our day to day lives it makes sense to learn as wide a range of skills as possible- a person who has had only one well paid but specialised job throughout their working life would be far less able to cope with being made redundant than somebody who has several smaller incomes earned from a variety of sources.