I appreciate that not everyone agrees with my view that a full Annex 13 investigation is neither required nor necessary. And if it was my relative or loved one involved, I too would want to know how/why.
This tragic event differs from MH17 and Pan Am 103 in that the causal event for both those needed to be established, whereas it is not the case for Tehran. Until the forensics were done, the exact cause of Pan Am 103's in-flight break up was unknown and there was sustained denial that a missile had downed MH17, so it was unsurprising that the states of occurrence went for an investigation in both cases. In its most cynical interpretation, you might consider that Iran opened an investigation as 'smoke on the objective' until it became obvious that the true cause could not be concealed. Or it may be that the people opening the investigation were unaware of the military involvement and were trying to do the right thing at the time. We will never become privy to that information.
Bear in mind also that there are dozens of Annex 13 investigations into fatal accidents around the world that have never seen the light of day, or which have taken so long that events have faded from memory other than for those with a personal connection. The fact that a safety investigation has been conducted does not mean its results will be made public, especially if it involves a state that consistently flouts international norms.
Despite the importance of 'face' in that part of the world, the Iranian authorities have admitted to a global audience that their military was responsible for the destruction of that aircraft and the enormous loss of life it entailed. They have admitted the engagement was caused by a failure within their command and control system (it matters not how many missiles were fired after the first one, which set the conditions and expectations for what followed) and they will no doubt learn the lessons from it. However, I find it inconceivable that a nation that considers itself beleaguered and militarily threatened by neighbours and others would willingly expose the inner workings and shortcomings of its air defence systems to public and international scrutiny, however much we might want them to do so. The context of this tragedy is important, and it will inevitably affect any inquest into events and the amount of information subsequently made available. You need to put yourself in Iranian shoes.
What matters now is preventing a recurrence. Let the legacy of this disaster be a more responsive system for alerting commercial traffic to areas of significantly increased risk. We do not need to know why, we just need to know when and where to avoid operating That implies a system for getting such information onto the flight deck for those already in the air, and into the flight planning system for everyone else. And it needs to be beyond the decision of operators and instead in the hands of regulators who can make the call unfettered by commercial considerations.