It would not have saved lives, that's obvious based on the fact that its an aircraft you are talking about.
Well it could have saved lives if it had been a sufficient deterrent to prevent the "hijacking" in the first place. And being able to track a plane could perhaps help to save lives on the ground. Can we agree that we would prefer to track a hijacked plane than not track it?
improved profiling and psychological monitoring of flightcrew.
It would be hard to think of a less effective measure than this. Psychologically profiling is unable to identify someone about to commit such an atrocity. No psychologist believes this is possible.
Due to the number of incidents, the best data probably comes from looking at mass murderers. Yes in retrospect people will often come out and say the offender was odd or unusual in some way, or had recently suffered a relationship breakdown. But that is very different to prospectively identifying someone in time to prevent an atrocity.
There are certainly some "risk factors" e.g. male gender, feeling isolated, being a bit aloof or awkward among others, recent marriage breakdown, financial problems etc. However these risk factors are so common they define a significant proportion of the population. Does anyone know a male pilot with marriage difficulties?
The risk factors have high sensitivity (i.e. some of these factors will be present in most offenders), but appalling specificity (in that the overwhelming majority of people with these risk factors will not offend). So we can use these risk factors to predict that the next offender will likely be a male with some interpersonal difficulties, but we can't predict the identity of the next offender. They are useful at a population level, but completely useless at an individual level.
This article is a few years old, but is still true today
Dark Matter: The Psychology Of Mass Murder
"Terrorism" used to be about getting a gun onto a plane and issuing demands to release prisoners etc. 9/11 changed that - it was about getting a pilot onto a plane, forcing entry to the deck and taking control. It has probably changed again with MH370.
I would certainly take my chances with being unable to isolate an electrical system than with psychological screening.