Most people, in a sense, "don't care" if there's a mask mandate at Gen Con. (They "don't care" in that, if there is one, they'll comply, because four days of gaming in masks beats four days sitting at home without them.)
Some people can't or won't come if there isn't a mask mandate. They're either at higher risk and can't justify the trip without some assurance that everyone is taking similar precautions, or they're just more risk-averse and don't want to spend all the time manning a vendor booth just to have shitty people come and blow their germs on everything. (Which, granted, happened pre-COVID, but now carries potential long-term serious consequences.)
Some people won't come if there *is* a mask mandate. They *don't* think that four days of gaming in masks beats staying home, and would stay home instead. (Many of) These people will shout about "personal choice" and "let people decide their own risks". In my opinion, they can make the "personal choice" to skip the con if they can't deal with a mask, but that's just me.
Personally, I'm mostly in the first camp, although I lean a little into the second. I think Gen Con *should* require masks, mostly because I personally don't care about excluding the third group vs. the second. Also, they've been saying they're required all year so far, so they should just make the policy concrete so all the people in the third group that haven't already made other plans can go ahead and do that well before the refund deadlines.
Also, the precautions for not getting/spreading COVID-19 at Gen Con are a lot bigger than just wearing masks or not and getting vaccinated or not. You should test beforehand (an at-home rapid test is probably fine if you're not symptomatic) to make sure you're not bringing it with you, and you should test again a few days after you get home to make sure you didn't bring it back and spread it to your friends, local game stores, and community at large. You should get vaccinated (if a doctor hasn't otherwise recommended against it). You should get a booster (again, if medically indicated for you). You should do all the personal hygiene stuff that gets stressed so much on these message board because we all know a significant portion of the attendees don't. (Although masking might also help with that smelly person next to you.)
Conventions like this are inherently risky right now, but there are things they can do to provide the most protection they can to the most attendees as possible, and it's downright irresponsible to not do them.