Fantasmic could be a show I would watch more than once every 10 years if they didn't have a theater that reached capacity. Getting there and having to sit on uncomfortable bleachers and wait for upwards of 2 hours is ridiculous, and then you can't just come and go as you please. So, I have to preface my response by saying I don't love Fantasmic.
That said, I'm really sorry you planned your day around seeing it and then were dissappointed that the theater reached capacity.
On the other hand, what did you expect the cast member you spoke with to do? The ideal response, I suppose, would be "you're right, American Idol is stupid and shouldn't affect Fantasmic, which is awesome, like it will, so I will cancel American Idol". But the cast member really is not empowered to do that. Nor are they empowered to ignore capacity. Capacity is not subjective, unfortunately. I suppose she could've said "I'll bring your complaint to management" and probably should've said that, but let's be honest... I dont' think that would change the outcome of the situation.
Not that I'm a fan of AI, but it does have everything to do with Disney in that it's owned by Disney. I think it's kinda unfair to lash out at AI because you didn't make it into your show on time, and it's unfair to lash out at the cast member whose job it was to do crowd control because she couldn't make an exception for you.
My advice is (and I only say this because it's the only way to make yourself happy)... show up early. Really early. And turn a blind eye to AI if it's not your thing. Maybe write a letter suggesting an alternative which would allow more showings of fantasmic.