I never said it was immutable. I said there are iconic elements to the series and the more iconic an element is the more it will be used. Some of the iconic elements get cycled out to shake things up, but the most iconic elements eventually get cycled back in. New ideas are introduced into the mix, and sometimes they also become iconic parts of the series. Different eras will have different elements that are on the table and that are off the table. The most famous and iconic elements are the ones that will be brought back to the table the most often.
Peter Parker being down on his luck is an element you should expect to frequently be on the table, as it has been for over six decades.