I work in aerospace and I have also experimented with thermite, and I my opinion is that if a thermite charge goes off in the cabin, bar the door, Katie. The aircraft is probably going to go down. Either the cabin interior will ignite, the thermite charge will burn through some flight-critical system, or both.
Never said it would be a good idea...
Just that if thermite burns down( and its the only way it is going to go) it would have to hit control or fuel system to disable the aircraft directly, and yes cabin fires are extremely bad (see Canary island 747 crash) but there have been improvements in fire retardation in the cabin of newer airliners.