Part of this is perception. Right now, Gen Con attendees have the perception that the Con is four days. That has been ingrained into our heads. ("The Best Four Days in Gaming", etc.) So now it has become almost routine for people to think of this as a four day event. Could it be longer? Absolutely! But it would take time to change the 4-day only perception that we have already built up in our minds. If Gen Con were to change to 7 days (for example) it would have to start changing the perception FAR in advance. Vendors would adapt. Game hosts would adapt. There are plenty of non-gaming events that are longer than four days that seem to do fine so I am pretty sure that Gen Con would do fine if it were to extend beyond the four days. For an example, if Gen Con were to go from Thursday to Tuesday and I was an exhibitor, I would plan for two crews to man the booth. One would work setup through Saturday, the other would work Sunday through Tear down with perhaps a manager type working Setup through Sunday and one Saturday through Tear Down. Game hosts could choose to run games/events the same way. Perhaps round one Thurs - Sunday, and Round 2 Sat through Tuesday. Attendees could stay all six days or just attend four days in a row. Passes could be daily, four day, or six day.
Still disagree about the vendors. For many game companies, the actual booth at the con is a money loser. You make money later thanks to publicity, but the con itself, you're in the red. Obviously not true for MtG dealers and the booths selling t-shirts and such. But true for many. You would be surprised at how many sighs of relief there are inside the dealer hall five minutes after close on Sunday. That last day, many just straight up don't want to be there anymore. It's stressful as ****. And while FFG might be able to have two entire staffs of booth workers (which I actually doubt, but maybe), smaller companies just can't afford that.
I seriously doubt any fifth day would come with a dealer hall. At *best*, maybe one set of dealers for part of a longer Con, and another set for the rest. But small companies just can't afford to go longer. Many would ditch Sunday right now if they could (but it's contractual, you can't).