One other reason I think GenCon made the announcement now, instead of waiting: folks still have time to cancel their tickets if the new guidelines are no longer palatable.
For those worried about dishonesty, or "breakthrough" infection, I totally get it. Everyone has their own level of risk tolerance, and a roomful of unmasked people, even vaccinated, is a lot to ask for many when we're still in the teeth of this thing.
However. IMO, the whole reason we locked down was to give the scientists time to get the vaccines out so COVID could become manageable. They've done that, and it's working: the vast majority of people getting hospitalized or dying now are the unvaccinated. To me, that makes the risk of attending in-person Gen Con worth the risk (although I'll still probably be masked up most of the time I'm inside, because it's still safer than not doing so). But I totally get anyone who decides the opposite and stays home!
I also think this updated policy will put less pressure on the volunteer staff, who now only have to confront people with neither a mask or wristband, which will hopefully be a lot smaller number than those who would have defied a general mask mandate. Maybe that's naive--I guess we'll see.