Maybe i'm just a cock-eyed optimist (which i'm not) but I don't see them getting that extreme that they'd even try and ban people from dressing up at cons. It'd be the very definition of killing the goose that lays the golden eggs. Plus, i'm not sure they have any legal recourse to ban that. Now, banning people who aren't authorized dealers from selling stuff - yeah. But, i'm not sure even that's legal unless they're selling knockoffs. I don't think they can sell me a legit costume (i.e. like buying it from anovos) and then ban me from being able to sell it.
I just don't see them being that stupid. That'd be the easiest way to get everyone to turn on marvel, star wars, disney itself, etc.
Oh I agree with you 100% that it would be completely stupid on their part and that it would, overall, hurt Disney in the long run. However, this is the same hyper-litigious company that forced a daycare to repaint their walls to remove Disney characters, even though the presence of the characters did not violate intellectual property rights because the daycare's income wasn't affected by the Disney characters.
Granted, for the longest time they have kept their litigiousness confined to strictly Disney characters and even that has not affected cosplayers unless it was an original Disney character (i.e. - Mickey, Minnie, Donald, etc... Snow White, Rapunzel, and other "Disney Princesses" don't usually count because they are based on preexisting stories).
However, from what I have been told by someone in the know (I'm not giving away their name), Disney has already tried to dictate where the 501st/Rebel Legion will be on a given weekend, even though it's an all-volunteer non-profit organization. Now the crackdown on cosplayers is probably speculation, and lightsabers won't really count unless it's a replica prop of one seen on-screen.
Given how litigious Disney is, though, it wouldn't surprise me if they come through and try to brute-force their will onto the public. What'll probably happen is that there will be such an outcry against them (especially if they mess with the 501st) that they will have no choice but to back down or face huge financial losses.