Mechanical back up is not a control law, it is just a mechanical back up. And even worse than that you have triple hydraulic failure, where the only means of control would be differential engine thrust, and maybe pax seats shifting...
My experience with non airbus is over 1,000 in Citation VII (direct mechanical flight control system) and a few in the B767. I know how a conventional airplane flies. I also have a few hours in the microsoft FS, which happens to be similar to A320 direct law. A joystick spring loaded to neutral and a trim wheel.
It is not that you are going to die because of flying in direct law, but there is risks.
And my point: it is NOT a conventional flight control system. Unless MS FS is... It is a crap of system. That is my point, and I will not willingly "revert" to it if I have a nice, closed loop, flight path stable, simple alternate law.