As Bruce Schneier put it (he's one of the world's top security gurus): "Any competent security expert can build a system so secure that he or she cannot think of any way of hacking it." But someone else will.
Even if the system isn't connected to anything else, it can be hacked by manufacture/maintenance/user subversion. The most dangerous security error is to imagine your system can't be hacked.
Yes I know who Bruce Schneier is, and yes I agree with the general statement that nothing is unhackable given enough time.
However the basic sanity test of viability still very much puts WiFi / remote control of a Boeing 777 firmly into the Hollywood fiction category. I doubt Mr Schneier would disagree with me there.