The greatest danger is not hacking the plane, but hacking the navigational systems that the plane relies on. It is trivially easy to hack signals from GPS, FAA NextGen (ADS-B), and ACARS.

The good thing is the redundancy of pilots in the cockpit. They know what the plane should be doing and where it is going, and will notice things like bank angle changes and altitude changes.

The In Flight Entertainment systems are not permitted to be connected to the flight control and management systems, or else the IFE must be certified to the level of the flight management/control systems. So there's an economic disincentive to connect the two.
Plus in IFE WiFi systems are completely airgapped from the rest of the airplane.

All that said, I have seen hacking systems that can even defeat an airgap to gain information (I have not seen one yet to push information), so I will never say that it cannot be done. It's just going to be very, very hard. Especially as there are so many versions of the various FMSs and FCSs in the wild, so you would have to attack one specific block of one specific model, with one specific carrier's specs.