A Typical strategy for breaking into a phone with no time limit is to simply wait until updates get published, examine what was fixed, and exploit the unpatched

A Typical strategy for breaking into a phone with no time limit is to simply wait until updates get published, examine what was fixed, and exploit the unpatched version of the phone. This is likely what happened in the FBI case you noted as there were a number of updates of ios9 published during that time. While iPhones are typically regarded as secure, the main problem with ios is that the source is closed and thus more likely to contain zero days.