I don't know whether I'm too forgiving, but with a place as large as WDW it must be logistically very difficult to monitor every last square inch and close all the gaps.
The thing that strikes me more than anything is how easy or at least possible it seems to be for people to get into the parks from the outside. Again, though, I don't know how simple it is to really seal off such large venues that thousands of workers have to get in an out of each day. I guess they could build giant border walls around the parks and restrict entry to a very limited number of entrances...? Still, I'm surprised it would be possible to get to something like Everest without anyone catching you is possible, particularly as it's not really near any external roads.
I'm not sure that blanketing the place in cameras would do much beyond perhaps being a deterrent. Cameras might be useful after the fact for prosecuting people, but the issue at the present seems to be more with people who film themselves breaking into the park and upload the footage without Disney apparently being able to do anything.