Mechanical and weather related failures are likely to diminish because government and industry are spending billions on safety measures. This is because the most significant trend in aviation is the inter-connectedness of systems. For instance, the control mechanism for a rudder is often connected to the cockpit by cable. It's easy to point to a severed cable in a crash investigation. Rudders in modern aircraft will be operated by electrical signals.
Signals filter through systems.
A glitch in one sector causes a glitch in another, sometimes making it impossible to determine what happened. The highly integrated systems and extensive cross-linking of aircraft systems make it hard to isolate failure. NTSB has to count the ways. Even when there's no trace of what occurred. The trend in aircraft capacity makes NTSB's work even more critical.