The viewpoint of the Apple vs. FBI showdown that says giving the key to one iPhone would provide a skeleton key to all iPhones has some credibility. In the development process, all iPhones are loaded with the exact same cookie-cutter code in order to save time and money, and therefore increase revenue. Translation: because the code loaded onto every iPhone in the world is exactly the same, breaking into the code on one provides a template to hack every iPhone on the planet.
Therefore, while the FBI does have a legitimate case, it needs to find another route to verifying the data on the San Bernardino shooter’s iPhone.