Does Apple mean that if a user forgets his password (for whatever reason), his iPhone would become useless forever? As an IT professional, I don't believe so at all.
Neither do I believe that if wrong passwords are entered several times then the data stored in that iPhone would be erased permanently, as mentioned in some news reports recently. If that's the case, iPhone would be the least safe smart phone--That means if your 6-year-old kid plays with your iPhone and tries passwords a few times while you are having a nap, the data in your iPhone could be gone forever.
Regarding the San Bernardino shooters' iPhone, Apple has the obiligation to assist FBI retrieving data in it. This is the obligation to the society. A locksmith should assist the police to unlock a criminal suspect's home, so should Apple.
Having said that, such data recovery work should be done on a case-by-case basis, authorized by court orders. The law enforcement should not have a dedicated hacking tool that allows them to hack into iPhones at their own wish. Don't forget that those working in law enforcement are also human beings. Some of them can break the law and abuse such hacking tool too.