Re: "For Your Protection", yadda yadda.
Nobody is asking Apple to assist in going on a fishing trip. Only the most pedantic of barrack room lawyers would argue that there is any question over the guilt of the alleged offenders in this case (an argument which rests only the fact of a lack of actual conviction at this stage. There is no challenge to the alleged facts of the case as far as I know).
And even if there were the FBI are asking for specific, very limited assistance in gathering information maintained by this one, specific, individual device. Nothing about what is being asked for has any implications for the privacy of anyone else.
The authorities already have far greater powers to investigate BEFORE the fact of a conviction, with the ability to seize documents, tap communications on the basis of probable cause, subject (in most cases I would like to think) to the issuance of a warrant. The obstacle in the case of an iPhone is a strictly technical matter. if the information were papers kept in a secure filing cabinet then the FBI would simply physically force that cabinet open and the implications for "the privacy of the rest of us" would not even be in question.
The only difference here is that the filing cabinet is an iPhone and the FBI need Apple's help to be able force it open IN A REASONABLE TIME. Crucially, they could do it without Apple's help, just potentially not in a timeframe that would make the resulting information useful.
Assuming a simple incremental brute-force approach of testing each passcode from 0000 to 9999 sequentially in turn, then they might get in within 10 seconds. But thanks to the mechanisms on the device, it could also take ~9999 hours and may take over a year to get in, by which time the information may no longer be of any use in PROTECTING the rest of us.
Or, they find that there is no information on which to act and we are all safe anyway.
Anyone proclaiming "PRIVACY IS AT STAKE" in this case is either an idiot or selling something.