What is the feasibility of fitting all aircraft with black boxes?
It would be very difficult to mandate new equipment into older aircraft. Look how much it took to mandate ADS-b. Mandating it into new or future built aircraft would be easier. We may try to incorporate the telemetry-like data we are getting now into some type of solution. Companies like Sentry and Stratus could design a portable unit that records all AHRS, GPS, traffic, cabin pressure altitude, and carbon monoxide presence data for less than $1000. Combine that with a personal electronic device like a tablet or smartphone and a patch cable or Bluetooth connector, and you have most of the capabilities of an FDR/CVR combo. It would not record the position of flight and engine controls, or engine performance data, however. 

The use of this may be easier to mandate. $1000 is fairly reasonable in aviation economics.

Making it mandatory for ALL passenger and property carrying commercial flights may be more feasible and likely. The federal government would implement it under the auspices of protection of the consumer or for the good of the public.