This is a good question. I want to echo something Sean said on twitter, that you quoted here, and I want to emphasize it because it is something I care a lot about personally.
As soon as Ironwood activates, you get verification of the current actual supply of ZEC from your full node.
You do not have to wait and see how many ZEC migrate from Orchard to Ironwood and you do not have to do game-theoretic reasoning about how an attacker might behave or how other legitimate users might behave in order to have this verification.
This is important to me, because my goal is to get the most outsiders to start trusting Zcash over the next ten years, and these outsiders can gain trust in Zcash much stronger and much quicker based on their full node verifying the supply directly, instead of based on game theory and based on observation of migrations out of Orchard.
This appears to be super confusing to almost everyone, because they are confusing two different things:
- No counterfeit coins were created.
- The current supply is sound (i.e. 16M ZEC now, 21M ZEC eventually).
These are different things!
I’m prioritizing the second one.
There are two universes we can be in right now
Universe 1: There was an unlimited amount of counterfeit ZEC generated inside the Orchard pool before the vulnerability was closed.
Universe 2: There was not.
The goal of Ironwood is that, regardless of whether we are in universe 1 or universe 2, in both cases everyone gets trustless verification that the supply is sound on Day 1 of Ironwood.
Universe 1: Ironwood activates → On Day 1 you can trustless verify that there are no more than 16M ZEC currently circulating. → Eventually we get a public verification from the turnstile that counterfeiting occurred in Orchard, but this doesn’t change the circulating supply, which is still 16M ZEC now, 21M ZEC eventually.
Universe 2: Ironwood activates → On Day 1 you can trustless verify that there are no more than 16M ZEC currently circulating. → Eventually we start to develop confidence that no counterfeiting ever occurred in Orchard, because less than the correct amount (4.5M ZEC) ever migrates out of Orchard, but this doesn’t change the fact that the circulating supply was correct and was verifiably correct as of Day 1.
Please let me know what you think of this. 
(P.S. There are a few technical caveats to this, and I’m happy to work through all the technical details with anyone in order to help highly technical people gain confidence in the truth of what I say above, but for most people those technical details are not necessary to understand in order to get correct, justified confidence in the truth of the above.)