The trials came usually very long time after those investigations results and recommendations.
The trial is there for punish any who (human .. or corporate ..etc ..) can be the culprit of the accident.
In any case the trial don't have such duty to improve safety.
So the two parts ... investigations and recommendations ... and trial are absolute necessity but are not complementary IMHO.
BTW .. you can't go free if you kill people .. it's immoral.
If I bend an airframe and a criminal investigation is likely I will claim the 5th and demand a lawyer. You will never know for sure what factors preceded the accident and, as sure as night follows day, it will happen again.
If on the other hand a no blame investigation wants to prevent a re-occurrence I would naturally be open and honest, even if it shows me to be stupid.
Only lawyers stand to benefit from criminal trials.