To use the classic XKCD comic, the crux of the case is that FBI is the one arguing the first panel (i.e., some bogus magical encryption we can never break), and because of that claim, they need to be able to compel Apple to compromise the security features using the old wrench trick.
The reality of there being practical alternatives for the FBI to pursue should give pause as to whether they can compel Apple to compromise the security features, and arguably the method described/discussed is indeed very practical.
All in all, it's less about the FBI's ability to do any of this and instead more about "should be the allowed to force a company to do something like this?". By demonstrating the claim that it's impossible to proceed without Apple's help is not true, I would think it should give pause to any court as to how to rule, since the implication of the ruling is pretty big.