So, terrorism? Probably less likely than anything Disney would cause through its own cutting of corners, weird members of various cults and literal Nazis who apparently have nothing to do with their lives other than stand outside WDW with signs notwithstanding.
I would like to believe that even with plenty of people angry with Disney for reasons both legitimate and stupid, and plenty of incompetence from those whose job is at least theoretically to hold potential attackers accountable, there's no way a person who tried to do such a thing would get away with it for any extended period of time. There's an above average chance that such activities would be extensively traced through Disney's own cameras or those in the pockets of 90% of their guests at any given time. Even with such levels of anger that some people are directing towards the company, I don't think most people would seriously consider going after Disney in such a way. Were they to do so, I could see a lot of people angry at Disney right now swing right back to Disney's corner should such an attack occur.
The Disney family hasn't really been involved with the company since Eisner took over (the ousted Ron Miller being Diane's husband). I'm sure they're not crazy about what the Disney company has done, but seeing as they've been, sad to say, basically irrelevant since the early 80s to what the company has been doing, I'm not sure why they're really coming into this. The only one who was really involved since then was Roy E, who's gone. Abigail Disney, Roy E.'s daughter, certainly likes to share opinions about TWDC but has never been involved in any capacity with the company, and I don't see any evidence that her views would go and encourage people to go and do a terrorism because of it. So sure, the Disney family may not like what Disney does. Plenty of people don't. But they're not exactly relevant to the actual running of the company today, and with few exceptions haven't been for longer than I've been alive.
Is there a way to totally prevent such a thing from ever happening? Probably not. But is there any real possibility of anything like that happening now? I'd give a definitive no.