A standby list would work, to a point, when you move to electronic ticketing for most events. People would sign up for a standby list for sold-out events. If folks return their e-tickets, people on the list would get a notification that the event was now available, and they could then go in & buy e-tickets. Since they don't need to show up with paper generics, it could work out. Logistics about order of notifications, time to purchase tickets before next person is notified, etc., are an exercise left to the user. :-)
AerodromeACE starts a "Open" list for our games starting day 1 .. we plan on no shows and add 2 extra seats at each of our 4 tables seating 12 at each table.
I do not think GenCon needs to change anything .. yes no shows hurt .. but add it into your planning , I have to say this year we did have a larger then normal number of no shows but thanks to Sunday players , we set up a 5th table to add 11 players that saved us lol.
Gencon goes above and beyond every year to assist us and I think they have a carried the fine line between Premier and non-premier fairly well.