Whilst it may seem overly harsh, it is designed to send a message to other teams not to do this again! I suppose the reason for this is effectively the stewards have decided that since the car failed to make it back to the pits, without force majeure, then that car failed scrutineering. As such, the only punishment that they could give was excluding the car from qualifying!
However.. I would say that I would be tempted to extend the ruling to all cars who fail to return to the pits under their own power. If they have mechanical problems, spin off and get beached, or some other reason, then they should start the race from the back of the grid. Qualifying is being treated as part of the race nowadays, so of the car fails, for sure allow repairs, but any repairs should either lead to starting at the back, or preferably from the pitlane!