Once all 3 chests have been earned via 6000 / 6000 Audience Approval everybody just stands around in the centre waiting for the thing to end so they can turn in the weekly. If they’ve already done the weekly they can literally just leave once they’ve got all of the chests.
There must be something Blizzard can do to make people want to stay around for the end of the event (if they want to, not make it mandatory). Right now there’s no benefit outside of the weekly.
Why not have an Amateur Actor’s Chest (green reward) repeatable every 2k audience approval or something?
In the case of the Theatre, they could, as you suggest, make it mandatory or add extra rewards for staying, or something else.
But the problem with any kind of what they are calling “Public Event” is simpler and deeper than this specific one.
When the event is new, it is well attended. Plenty of people. The rewards are worth having, and there are maybe achievements.
But attendance falls off. And then it may be impossible to solo, or, if they stretched to make it possible, it will be unpleasant - and the achievements will be unattainable.
“Public Events” were a fundamentally bad idea, that cannot be made good. Pity.