Well, I'd say there were two sure facts about the outcome of the Duggan inquiry before it even started, and these were
a) The verdict would almost certainly be "Mistakes were made but everyone acted in good faith and they all emerge with their integrity intact, except maybe for the dead guy", because that's usually the result of just about every inquiry into public officials major and minor, and
b) Huge numbers of people were then going to go apeshit about it along pretty predictable lines.
Now, I'm reflexively inclined towards scepticism for the authorities on issues like the police capping civilians in the street, not least because we've had enough demonstrations recently of how major organisations, public bodies and businesses close ranks in a crisis.
What I will say is that the immediate "It's a stitch-up!" response from much of the public is understandable, given the number of killings, stadium disasters and official assaults on the citizenry that were dismissed as mere mistakes made in good faith or the paranoia of extreme political groups at the time, only to emerge as the worst kind of criminal skullduggery thirty years later, once everyone involved is safely resigned or dead.
I mean, look - if the authorities are capable of sliming out of culpability for a Hillsborough disaster or for an all-out militarised assault on the miners or a three-way conspiracy against the nation like the News International scandal, only for the ugly truth to be dragged out years later, that's going to encourage a lot of people to look askance at a Duggan inquiry, or a Chilcot one for that matter.
It's not that every controversy involving the police or the government is necessarily a conspiracy against the public interest. It's more that even a vague grasp on the history of similar events should naturally incline the citizenry towards a healthy suspicion, in our own interest if nothing else.