18 wheeler
I understand what you are suggesting. The problem is turbine helicopter engines are designed drive the gearbox. The "jetblast" is usually directed towards an area where it will cause minimum problems to ground operations & not necessarily directly rearwards as you would need for propulsion.
You could of course design a gear box where you disconnect the drive from the main transmission & direct is to a big propellor on the back as with an autogyro. However if you are going to these lengths it would probably be better just to have a reseve tail rotor.
As with any design the risk of system failures is evaluated against cost & weight. Having a back up system for the tailrotor would be a huge penalty for a very rare failure.