If FBW were introduced then the entire intake and exhaust management systems would need to be reconfigured, more sensors and relays required etc. . Should an engine misfire for some reason, the computer management system may shut down the engine. This happens quite a bit now days in motor vehicles built after 1990. It would no longer be an easy task to identify the cause, especially by you the owner.
The aircraft would have to be hooked up to a scan tool to detect the fault code and then the fault traced back and rectified. Considerable knowledge of the system and components would be required to identify and rectify a fault, often requiring removal of a component and testing with a new component.
Becomes very expensive, especially if the tech doing the work has no used parts he can try, and you as the end user will pay for all the components fitted to fault find whether they were the cause or not. I could go on about this but cutting it short, it would turn a relatively cheap excerise of fault finding into a major cost. (EG: 2 yr old Mercedes shuts down. Roadside services can't assist. Vehicle transported to Mercedes dealer. Dealer has vehicle for 3 weeks whilst trying to track cause of fuel system shutting down. Nobody in Aus can figure it out. $100,000 equipment shipped to Aus from Germany. Tests completed. No finding fault. Another tech guy sent out from Germany and off the top of his head at an inspiration, he checks a mercury switch (installed to prevent fuel flow in case of roll over) under the dash. Switch mount has somehow come loose and switch is closed. Fuel system shut down. This was not indicated by the software diagonsis.) Imagine the cost. In this case to Mercedes but caused a lot of headaches for all concerned.
My thoughts are stick with easy methods until we're absolutely sure we can trust computerised systems which can identify cause of a problem to the actual failing component.